 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued March 22, 2019                  Decided April 1, 2019

                        No. 19-5042

                  DAMIEN GUEDES, ET AL.,
                      APPELLANTS

                             v.

BUREAU OF ALCOHOL, TOBACCO, FIREARMS AND EXPLOSIVES,
                      ET AL.,
                     APPELLEES


                 Consolidated with 19-5044


        Appeals from the United States District Court
                for the District of Columbia
                    (No. 1:18-cv-02988)
                    (No. 1:18-cv-03086)




   Erik S. Jaffe argued the cause for appellants Damien
Guedes, et al. With him on the briefs were Joshua Prince and
Adam Kraut.

     Stephen D. Stamboulieh and Alan Alexander Beck were on
the brief for appellants David Codrea, et al.
                              2
     Ilya Shapiro was on the brief for amicus curiae Cato
Institute in support of appellants.

    Conor Shaw and Nikhel S. Sus were on the brief for amici
curiae Citizens for Responsibility and Ethics in Washington
and Former Government and Ethics Officials in support of
appellants.

     Steven M. Simpson was on the brief for amici curiae The
New Civil Liberties Alliance and W. Clark Aposhian in support
of appellants.

   J. Carl Cecere, Jr. was on the brief for amicus curiae
Morton Rosenberg in support of reversal.

    Bradley Hinshelwood, Attorney, and Hashim M.
Mooppan, Deputy Assistant Attorney General, U.S.
Department of Justice, argued the causes for appellees. With
them on the brief were Matthew J. Glover, Counsel to the
Assistant Attorney General, and Scott R. McIntosh, Michael S.
Raab, and Abby C. Wright, Attorneys.

     Ian Simmons, Matt Schock, and Anthony G. Beasley were
on the brief for amicus curiae Giffords Law Center to Prevent
Gun Violence in support of appellees.

    Before: HENDERSON, SRINIVASAN and MILLETT, Circuit
Judges.

    Opinion for the Court filed PER CURIAM.

    Opinion concurring in part and dissenting in part filed by
Circuit Judge HENDERSON.
                               3
     PER CURIAM: In October 2017, a lone gunman armed with
bump-stock-enhanced semiautomatic weapons murdered 58
people and wounded hundreds more in a mass shooting at a
concert in Las Vegas, Nevada. In the wake of that tragedy, the
Bureau of Alcohol, Tobacco, Firearms and Explosives
(“Bureau”) promulgated through formal notice-and-comment
proceedings a rule that classifies bump-stock devices as
machine guns under the National Firearms Act, 26 U.S.C.
§§ 5801–5872. See Bump-Stock-Type Devices, 83 Fed. Reg.
66,514 (Dec. 26, 2018) (“Bump-Stock Rule”). The then-
Acting Attorney General Matthew Whitaker initially signed the
final Bump-Stock Rule, and Attorney General William Barr
independently ratified it shortly after taking office. Bump-
stock owners and advocates filed separate lawsuits in the
United States District Court for the District of Columbia to
prevent the Rule from taking effect. The district court denied
the plaintiffs’ motions for a preliminary injunction to halt the
Rule’s effective date. Guedes v. Bureau of Alcohol, Tobacco,
Firearms, and Explosives, 356 F. Supp. 3d 109 (D.D.C. 2019).
We affirm the denial of preliminary injunctive relief.

                               I

                               A

     The National Firearms Act (i) regulates the production,
dealing in, possession, transfer, import, and export of covered
firearms; (ii) creates a national firearms registry; and (iii)
imposes taxes on firearms importers, manufacturers, and
dealers, as well as specified transfers of covered firearms. 26
U.S.C. §§ 5801–5861. Failure to comply with the National
Firearms Act’s requirements results in penalties and forfeiture,
and subjects the violator to the general enforcement measures
available under the internal revenue laws. Id. §§ 5871–5872.
                               4
     The firearms subject to regulation and registration under
the National Firearms Act include “machinegun[s].” 26 U.S.C.
§ 5845(a).1 The statute defines a “machinegun” as “any
weapon which shoots, is designed to shoot, or can be readily
restored to shoot, automatically more than one shot, without
manual reloading, by a single function of the trigger.” 26
U.S.C. § 5845(b). The definition also covers “the frame or
receiver of any such weapon,” as well as “any part” or
“combination of parts designed and intended, for use in
converting a weapon into a machinegun,” and “any
combination of parts from which a machinegun can be
assembled” as long as those “parts are in the possession or
under the control of a person.” Id.

    Congress expressly charged the Attorney General with the
“administration and enforcement” of the National Firearms
Act, 26 U.S.C. § 7801(a)(1), (a)(2)(A), and provided that the
Attorney General “shall prescribe all needful rules and
regulations for the enforcement of” the Act,” id. § 7805; see id.
§ 7801(a)(2)(A).

     The Gun Control Act of 1968, 18 U.S.C. § 921 et seq., as
amended by the Firearm Owners’ Protection Act, Pub. L. No.
99-308, 100 Stat. 449 (1986), imposes both a regulatory
licensing scheme and criminal prohibitions on specified
firearms transactions. See 18 U.S.C. § 923 (licensing scheme);
id. § 922 (criminal prohibitions). The Gun Control Act
incorporates by reference the definition of machine gun in the
National Firearms Act, 26 U.S.C. § 5845(b). See 18 U.S.C.
§ 921(a)(23). The Gun Control Act also expressly delegates
administrative and rulemaking authority to the Attorney
General to “prescribe only such rules and regulations as are

1
 Except when quoting sources, we use the two-word spelling of
machine gun.
                               5
necessary to carry out the provisions of this chapter.” Id.
§ 926(a).

   The Attorney General has delegated the responsibility for
administering and enforcing the National Firearms Act and the
Gun Control Act to the Bureau. See 28 C.F.R. § 0.130(a).

                               B

                               1

    Machine guns are generally prohibited by federal law. See
18 U.S.C. § 922(o). On the other hand, many firearms that
require a distinct pull of the trigger to shoot each bullet are
lawful. See generally id. § 922; 26 U.S.C. § 5845.

     A “bump stock” is a device that replaces the standard
stationary stock of a semiautomatic rifle—the part of the rifle
that typically rests against the shooter’s shoulder—with a non-
stationary, sliding stock that allows the shooter to rapidly
increase the rate of fire, approximating that of an automatic
weapon. 83 Fed. Reg. at 66,516. A bump stock does so by
channeling and directing the recoil energy from each shot “into
the space created by the sliding stock (approximately 1.5
inches) in constrained linear rearward and forward paths.” Id.
at 66,518. In so doing, the bump stock “harnesses the firearm’s
recoil energy as part of a continuous back-and-forth cycle that
allows the shooter to attain continuous firing” following a
single pull of the trigger. Id. at 66,533. That design allows the
shooter, by maintaining constant backward pressure on the
trigger as well as forward pressure on the front of the gun, to
fire bullets continuously and at a high rate of fire to “mimic”
the performance of a fully automatic weapon. Id. at 66,516.
                               6
     Exercising his regulatory authority, the Attorney General
first included a bump-stock type device within the statutory
definition of “machinegun” in 2006. See ATF Ruling 2006-2;
see also Akins v. United States, 312 F. App’x 197, 199 (11th
Cir. 2009) (summary order). In later years, some other bump-
stock devices were not categorized as machine guns. 83 Fed.
Reg. at 66,514.

                               2

     On October 1, 2017, a shooter used multiple
semiautomatic rifles equipped with bump stocks to fire several
hundred rounds of ammunition into a crowd of concert
attendees within a roughly ten-minute span of time. The
“‘rapid fire’ operation” of the shooter’s weapons enabled by
the bump stocks left 58 dead and approximately 500 wounded.
83 Fed. Reg. at 66,516.

    The Las Vegas massacre prompted an immediate outcry
from the public and members of Congress. See Guedes, 356 F.
Supp. 3d at 120, 123. In response, President Trump “direct[ed]
the Department of Justice, * * * as expeditiously as possible, to
propose for notice and comment a rule banning all devices that
turn legal weapons into machineguns.” Application of the
Definition of Machinegun to “Bump Fire” Stocks and Other
Similar Devices, 83 Fed. Reg. 7,949, 7,949 (Feb. 20, 2018).
The Bureau then revisited the status of bump stocks and
addressed the variation in its prior positions. 83 Fed. Reg. at
66,516–66,517. On March 29, 2018, then-Attorney General
Sessions issued a Notice of Proposed Rulemaking that
suggested “amend[ing] the Bureau of Alcohol, Tobacco,
Firearms, and Explosives regulations to clarify that [bump-
stock-type devices] are ‘machineguns’” under 26 U.S.C.
§ 5845(b). See Bump-Stock-Type Devices, 83 Fed. Reg.
13,442 (March 29, 2018).
                               7
     The Bureau promulgated its final rule on December 26,
2018. With respect to the statutory definition of machine gun,
the Bump-Stock Rule provided that the National Firearms
Act’s use of “the term ‘automatically’ as it modifies ‘shoots, is
designed to shoot, or can be readily restored to shoot,’” 26
U.S.C. § 5845(b), “means functioning as the result of a self-
acting or self-regulating mechanism that allows the firing of
multiple rounds through a single function of the trigger.” 83
Fed. Reg. at 66,553–66,554 (codified at 27 C.F.R. §§ 447.11,
478.11, 479.11). The Rule further defined “single function of
the trigger,” 26 U.S.C. § 5845(b), to mean “a single pull of the
trigger and analogous motions.” 83 Fed. Reg. at 66,553–
66,554 (codified at 27 C.F.R. §§ 447.11, 478.11, 479.11).

     In light of those definitions, the Bump-Stock Rule
concluded that the statutory term “‘machinegun’ includes a
bump-stock-type device”—that is, “a device that allows a
semiautomatic firearm to shoot more than one shot with a
single pull of the trigger by harnessing the recoil energy of the
semiautomatic firearm to which it is affixed so that the trigger
resets and continues firing without additional physical
manipulation of the trigger by the shooter.” 83 Fed. Reg. at
66,553–66,554 (codified at 27 C.F.R. §§ 447.11, 478.11,
479.11).

     In adopting the Bump-Stock Rule, the Bureau relied on
both the “plain meaning” of the statute and the agency’s charge
to implement the National Firearms Act and the Gun Control
Act. 83 Fed. Reg. at 66,527 (citing and invoking Chevron
U.S.A. v. Natural Res. Def. Council, 467 U.S. 837 (1984)). The
Bureau explained that the Bump-Stock Rule both “accord[s]
with the plain meaning” of the statute, and “rests on a
reasonable construction of” any “ambiguous” statutory terms.
Id. In the Bureau’s view, by not further defining the terms
“automatically” and “single function of the trigger,” Congress
                                8
“left it to the [Attorney General] to define [them] in the event
those terms are ambiguous.” Id. (citing Chevron, 467 U.S. at
844); see also id. at 66,515 (citing delegations of regulatory
authority under 26 U.S.C. §§ 7801(a)(2)(A), 7805(a), and 18
U.S.C. § 926(a)).

     The Bureau was explicit that the Bump-Stock Rule would
only become “effective” on March 26, 2019, ninety days after
promulgation. 83 Fed. Reg. at 66,514. The Bureau further
assured that individuals would be subject to “criminal liability
only for possessing bump-stock-type devices after the effective
date of regulation, not for possession before that date.” Id. at
66,525; see also id. (providing that the Rule “criminalize[s]
only future conduct, not past possession of bump-stock-type
devices that ceases by the effective date”); id. at 66,539 (“To
the extent that owners timely destroy or abandon these bump-
stock-type devices, they will not be in violation of the law[.]”).
Bump-stock owners were directed to destroy their devices or
leave them at a Bureau office by March 26, 2019. Id. at 66,514.

     Although most of the rulemaking process occurred during
the tenure of Attorney General Jefferson Sessions, he resigned
his office on November 7, 2018. The President then invoked
the Federal Vacancies Reform Act of 1998 (“Reform Act”), 5
U.S.C. § 3345(a)(3), to designate Matthew Whitaker, who had
been Sessions’ chief of staff, “to perform the functions and
duties of the office of Attorney General, until the position is
filled by appointment or subsequent designation.”
Memorandum from President Donald Trump to Matthew
George Whitaker, Chief of Staff, Department of Justice (Nov.
8, 2018), J.A. 277. The final Bump-Stock Rule was signed by
then-Acting Attorney General Whitaker. Whitaker served as
the Acting Attorney General for 98 days, until William Barr
was sworn in as the Attorney General on February 14, 2019.
                                9
See Bump-Stock-Type Devices, 84 Fed. Reg. 9,239, 9,240
(March 14, 2019).

     On March 11, 2019, Attorney General Barr announced that
he had “independently reevaluate[d]” the Bump-Stock Rule
and the “underlying rulemaking record.” 94 Fed. Reg. at 9,240.
“[H]aving reevaluated those materials without any deference to
[Whitaker’s] earlier decision,” Attorney General Barr
“personally c[a]me to the conclusion that it is appropriate to
ratify and affirm the final rule,” and did so. Id.

                                C

     Three groups of bump-stock owners and advocates filed
suit in the United States District Court for the District of
Columbia to prevent the Bump-Stock Rule from taking effect.
See Damien Guedes v. Bureau of Alcohol, Tobacco, Firearms,
and Explosives, No. 18-cv-2988; David Codrea v. William P.
Barr, No. 18-cv-3086; Firearms Policy Coalition, Inc. v.
William P. Barr, No. 18-cv-3083. As relevant here, the Guedes
plaintiffs (“Guedes”) and the Codrea plaintiffs (“Codrea”)
argued that the Bureau promulgated the Bump-Stock Rule in
violation of the Administrative Procedure Act, 5 U.S.C. § 500
et seq. Also, the Firearms Policy Coalition (“Coalition”) and
Codrea argued that Acting Attorney General Whitaker lacked
the legal authority to promulgate the Rule because his
designation as Acting Attorney General violated the Attorney
General Act, 28 U.S.C. § 508, and the Appointments Clause of
the Constitution, Article II, Section 2, Clause 2.

     The district court denied all three motions for a
preliminary injunction. Guedes, 356 F. Supp. 3d at 119. The
district court concluded that Guedes, Codrea, and the Coalition
had not demonstrated a likelihood of success on the merits.
The court first held that “[m]ost of the plaintiffs’ administrative
                              10
law challenges are foreclosed by the Chevron doctrine,” and
the Rule “adequately explained” the agency’s decision to
classify bump-stock-type devices as machine guns. Id. at 120.
As to the challenges to Whitaker’s authority, the district court
held that the Reform Act permits the President to deviate from
the line of succession that the Attorney General Act provides,
subject to certain statutory limitations that indisputably were
satisfied with Whitaker’s appointment. Guedes, 356 F. Supp.
3d at 120–121. The court also rejected the Coalition’s and
Codrea’s Appointments Clause challenge as “foreclosed by
Supreme Court precedent and historical practice.” Id. at 121.

     Guedes, Codrea, and the Coalition all appealed. But none
of them sought a stay or an injunction pending appeal. They
chose instead to seek highly expedited disposition, which this
court granted. While the appeal was pending, Attorney
General Barr ratified and individually endorsed the final
Bump-Stock Rule. At the post-argument request of the
Coalition, we voluntarily dismissed its appeal. Order, Guedes
v. Bureau of Alcohol, Tobacco, Firearms, and Explosives, No.
19-5042 (March 23, 2019) (per curiam). But because Codrea
presses the same challenge to Whitaker’s authority to
promulgate the Rule as the Coalition had raised, Codrea Br.
20–21, that issue remains before us in reviewing the district
court’s denial of a preliminary injunction.

                              II

     A preliminary injunction is “an extraordinary remedy that
may only be awarded upon a clear showing that the plaintiff is
entitled to such relief.” Winter v. Natural Res. Def. Council,
Inc., 555 U.S. 7, 22 (2008). The plaintiffs bear the burden of
persuasion in seeking preliminary relief. Cobell v. Norton, 391
F.3d 251, 258 (D.C. Cir. 2004). Specifically, Guedes and
Codrea must establish that: (1) they are “likely to succeed on
                                11
the merits”; (2) they are “likely to suffer irreparable harm in the
absence of preliminary relief”; (3) the “balance of equities” tips
in their favor; and (4) “an injunction is in the public interest.”
Winter, 555 U.S. at 20; accord Aamer v. Obama, 742 F.3d
1023, 1038 (D.C. Cir. 2014). The last two factors “merge when
the Government is the opposing party.” Nken v. Holder, 556
U.S. 418, 435 (2009).

     We review a district court’s denial of a preliminary
injunction for an abuse of discretion, but in doing so we review
the district court’s legal conclusions de novo and any findings
of fact for clear error. Serono Labs., Inc. v. Shalala, 158 F.3d
1313, 1318 (D.C. Cir. 1998).

                               III

     A foundational requirement for obtaining preliminary
injunctive relief is that the plaintiffs demonstrate a likelihood
of success on the merits. See Nken, 556 U.S. at 434 (“The first
two factors of the traditional standard [i.e., likelihood of
success on the merits and irreparable injury] are the most
critical.”); Sherley v. Sebelius, 644 F.3d 388, 393 (D.C. Cir.
2011) (raising the possibility that “likelihood of success is an
independent, free-standing requirement for a preliminary
injunction”) (quoting Davis v. Pension Benefit Guar. Corp.,
571 F.3d 1288, 1296 (D.C. Cir. 2009) (Kavanaugh, J.,
concurring)).

     Neither the challenge to Acting Attorney General
Whitaker’s authority nor the objections to the substantive
validity of the Bump-Stock Rule clears that hurdle. And
because the plaintiffs have shown no likelihood of success on
the merits, we choose not to “proceed to review the other three
preliminary injunction factors.” Arkansas Dairy Coop. Ass’n
                              12
v. United States Dep’t of Agric., 573 F.3d 815, 832 (D.C. Cir.
2009).

                               A

     Codrea levels a broadside attack on the rule as
categorically invalid because Acting Attorney General
Whitaker allegedly lacked the legal authority to approve the
Bump-Stock Rule’s issuance. Specifically, Codrea argues that
Whitaker’s designation to serve as Acting Attorney General
violated both the Attorney General Act, 28 U.S.C. § 508, and
the Constitution’s Appointments Clause, U.S. Const. Art. II,
§ 2, cl. 2. Whether or not those arguments would otherwise
have had merit (something we do not decide), Codrea has no
likelihood of success on this claim because the rule has been
independently ratified by Attorney General William Barr,
whose valid appointment and authority to ratify is
unquestioned.
     The Appointments Clause requires that “all * * * Officers
of the United States” be appointed by the President “by and
with the Advice and Consent of the Senate.” U.S. Const. Art.
II, § 2, cl. 2. This requirement is the “default manner of
appointment,” Edmond v. United States, 520 U.S. 651, 660
(1997), with the only exception being that Congress may vest
the appointment of “inferior Officers” in “the President alone,”
“Courts of Law,” and “the Heads of Departments,” U.S. Const.
Art. II, § 2, cl. 2 .
     One stark consequence of this scheme is that “the
responsibilities of an office * * * [can] go unperformed if a
vacancy arises and the President and Senate cannot promptly
agree on a replacement.” National Labor Relations Bd. v. SW
Gen., Inc., 137 S. Ct. 929, 934 (2017); Buckley v. Valeo, 424
U.S. 1, 132 (1976) (per curiam) (“[A]ll officers of the United
States are to be appointed in accordance with the Clause.”).
                               13
“Since the beginning of the nation,” Congress has addressed
this problem through “vacancy statutes” that grant the
President the authority to designate acting officials to “keep the
federal bureaucracy humming.” SW General, Inc. v. National
Labor Relations Bd., 796 F.3d 67, 70 (D.C. Cir. 2015)
(quotation marks omitted), aff’d, 137 S. Ct. 929 (2017).
     The Reform Act is the most recent iteration of that
interbranch accommodation. It provides for three options
whenever a Senate-confirmed officer “dies, resigns, or is
otherwise unable to perform the functions and duties of the
office[.]” 5 U.S.C. § 3345(a). The default is for the “first
assistant” to take the helm. Id. § 3345(a)(1). But the Reform
Act allows the President to choose another person instead, as
long as that person is either a Senate-confirmed appointee, id.
§ 3345(a)(2), or an employee within the same agency, subject
to certain duration-of-service and pay-scale requirements, id.
§ 3345(a)(3). Mr. Whitaker was designated under the latter
option, since his service as chief of staff comported with the
Reform Act’s duration-of-service and pay grade requirements.
Guedes, 356 F. Supp. 3d at 138 (“The parties do not dispute
that Whitaker satisfies the eligibility criteria in the [Reform
Act.]”).

     Congress broadly designated the Reform Act to be the
“exclusive means for temporarily authorizing an acting official
to perform the functions and duties of any” Executive office
that would otherwise require Senate confirmation. 5 U.S.C.
§ 3347(a). But there is an “unless”—Congress crafted
exceptions to that exclusivity. Id. As relevant here, Section
3347(a) does not control if another “statutory provision
expressly * * * designates an officer or employee to perform
the functions and duties of a specified office temporarily in an
acting capacity[.]” Id. § 3347(a)(1)(B).
                                14
     The Attorney General Act, 28 U.S.C. § 508, is one of those
office-specific vacancy statutes. That statute specifies a line of
succession for a vacancy in the Office of the Attorney General.
First in line is the Deputy Attorney General, who “may exercise
all the duties of th[e] office” and who, “for the purpose of
section 3345 of [the Reform Act],” is deemed “the first
assistant to the Attorney General.” 28 U.S.C. § 508(a). If the
Deputy Attorney General is unavailable, the Attorney General
Act directs that “the Associate Attorney General shall act as
Attorney General,” and “[t]he Attorney General may designate
the Solicitor General and the Assistant Attorneys General, in
further order of succession, to act as Attorney General.” Id.
§ 508(b).

     Codrea and the Department have battled at length over the
interaction between the Reform Act and the Attorney General
Act in the event of a vacancy in the position of the Attorney
General. The Government maintains, and the district court
agreed, that the two statutes provide the President with
alternative means of designating an acting replacement.
Guedes, 356 F. Supp. 3d at 139; Gov’t Br. 40–58. Codrea, by
comparison, reads the Attorney General Act as the exclusive
path for designating an acting Attorney General, with the
Reform Act available only after the line of succession in the
Attorney General Act has been exhausted. Codrea Br. 20–21
(incorporating Coalition Br. 6). Codrea also argues that the
designation of a mere employee to perform the duties of a
principal office like that of the Attorney General, even on an
acting basis, raises substantial constitutional questions, at least
when no exigency requires that designation. Id. (adopting
Coalition Reply Br. 15).
    We need not wade into that thicket. While this appeal was
pending, Attorney General Barr independently “familiarized
[him]self with the rulemaking record [and] * * * reevaluated
those materials without any deference to [Whitaker’s] earlier
                                15
decision.”     84 Fed. Reg. at 9,240.         Following this
“independent[] reevaluat[ion] [of] the * * * rule and the
underlying rulemaking record,” Attorney General Barr
“personally c[a]me to the conclusion that it [wa]s appropriate
to ratify and affirm the final rule.” Id.
     Codrea accepts the validity of Attorney General Barr’s
ratification as to both his statutory and his Appointments
Clause claims. Codrea Br. 20–21 (adopting Coalition Reply
Br. 22). See also 5 U.S.C. § 3348(d)(1)–(2) (only prohibiting
the ratification of nondelegable duties); 28 U.S.C. § 510
(authorizing delegation of “any function of the Attorney
General”). And with that act of ratification and the concession,
Codrea’s likelihood of success on the merits of his challenge to
the rule based on Acting Attorney General Whitaker’s role in
its promulgation reduces to zero.
    Codrea insists otherwise. He argues that Attorney General
Barr’s ratification does not moot the claim because of the
mootness doctrine’s exceptions for a defendant’s voluntary
cessation of challenged conduct or for acts capable of repetition
yet evading review. Codrea Br. 20–21 (adopting Coalition
Reply Br. 17). That argument fails because ratification is
generally treated as a disposition on the legal merits of the
appointments challenge and, in any event, no mootness
exception applies in this case.
                                1
     The mootness doctrine “ensures compliance with Article
III’s case and controversy requirement by ‘limit[ing] federal
courts to deciding actual, ongoing controversies.’” Aref v.
Lynch, 833 F.3d 242, 250 (D.C. Cir. 2016) (quoting American
Bar Ass’n v. FTC, 636 F.3d 641, 645 (D.C. Cir. 2011)). A case
is moot if our decision will neither “presently affect the parties’
rights nor have a more-than-speculative chance of affecting
                               16
them in the future.” Id. (internal quotation marks omitted)
(quoting American Bar Ass’n, 636 F.3d at 645).
     We have repeatedly held that a properly appointed
official’s ratification of an allegedly improper official’s prior
action, rather than mooting a claim, resolves the claim on the
merits by “remedy[ing] [the] defect” (if any) from the initial
appointment. Wilkes-Barre Hosp. Co. v. National Labor
Relations Bd., 857 F.3d 364, 371 (D.C. Cir. 2017). This is so
regardless of whether “the previous [officer] was” or was not
“validly appointed under either the Vacancies Act or the
Appointments Clause.” Intercollegiate Broad. Sys. Inc. v.
Copyright Royalty Bd., 796 F.3d 111, 119 n.3 (D.C. Cir. 2015)
(ratification defeats Appointments Clause challenge) (citing
Doolin Sec. Sav. Bank, F.S.B. v. Office of Thrift Supervision,
139 F.3d 203, 205, 207, 212–214 (D.C. Cir. 1998), superseded
by statute on other grounds, Federal Vacancies Reform Act of
1998, Pub. L. No. 105-277, 122 Stat. 2681, as recognized in
SW Gen., Inc., 796 F.3d at 70–71); FEC v. Legi-Tech, Inc., 75
F.3d 704, 706, 708–710 (D.C. Cir. 1996) (similar).
     In Doolin, we treated the curative effects of ratification as
analogous to rendering any defect in the agency’s action
“harmless error” under the Administrative Procedure Act, 5
U.S.C. § 706. 139 F.3d at 212. So viewed, ratification purges
any residual taint or prejudice left over from the allegedly
invalid appointment. Legi-Tech, 75 F.3d at 708 n.5 (“[T]he
issue is not whether Legi-Tech was prejudiced by the original
[decision], which it undoubtedly was, but whether, given the
FEC’s remedial actions, there is sufficient remaining prejudice
to warrant dismissal.”); Intercollegiate Broad., 796 F.3d at 124
(citing Legi-Tech for the same proposition). When viewed as
analogous to harmless-error analysis, ratification is treated as
resolving the merits of the challenger’s claim in the agency’s
favor. Cf. Doolin, 139 F.3d at 212; Combat Veterans for Cong.
Political Action Comm. v. FEC, 795 F.3d 151, 157 (D.C. Cir.
                              17
2015) (rejecting a procedural challenge to a Federal Election
Commission fine on the merits because the alleged infirmity
produced no “prejudice”).
     Those cases’ treatment of ratification as resolving the
merits of a claimed appointment flaw parallels how this court
analyzes the agency practice of post-promulgation notice and
comment. When an agency “issues final regulations without
the requisite comment period and then tries to cure that
Administrative Procedure Act violation by holding a post-
promulgation comment period,” we have repeatedly held that
the agency prevails on the merits as long as it can demonstrate
that it has kept an “open mind” throughout the subsequent
comment period. See, e.g., Intermountain Ins. Serv. of Vail v.
Commissioner, 650 F.3d 691, 709 (D.C. Cir. 2011) (emphasis
added), vacated and remanded on other grounds, 566 U.S. 972
(2012), dismissed on unopposed motion, No. 10-1204, 2012
WL 2371486, at *1 (D.C. Cir. June 11, 2012); Advocates for
Highway & Auto Safety v. Federal Highway Admin., 28 F.3d
1288, 1291–1293 (D.C. Cir. 1994) (same).
     Codrea points to Landry v. FDIC, 204 F.3d 1125 (D.C.
Cir. 2000), in which this court resolved the merits of an
Appointments Clause challenge to an administrative law
judge’s decision, notwithstanding the subsequent de novo
review and affirmance of that decision by the agency itself, id.
at 1131. That case is of no help to Codrea. Landry carved out
a narrow exception to ratification’s curative effect for
Appointments Clause challenges to the acts of “purely decision
recommending employees.” Id. at 1131–1132. This court
explained that, if ratification were an escape hatch in those
cases, “then all such arrangements would escape judicial
review” because the challenged ALJ action would never obtain
judicial review without first exhausting that ratifying internal
agency review process. Id. Only when that particular “catch-
22” is present does the Landry approach apply. Id.; accord
                               18
Intercollegiate Broad., 796 F.3d at 124 (distinguishing Landry
on that basis). The succession of a Presidentially appointed
and Senate-confirmed Attorney General does not remotely
implicate the Landry scenario.
                                2
     Codrea argues that we should analyze the effect of
ratification through the lens of mootness rather than treating
ratification as resolving the case on the merits. Codrea Br. 20–
21 (adopting Coalition Reply Br. 16–17).
     Codrea notes that all of our prior ratification cases dealt
with appointments challenges that arose as defenses to
enforcement actions that were being prosecuted by a properly
appointed official, but that were allegedly “tainted” by some
preceding action of an unlawfully appointed official. Codrea
Br. 20–21 (adopting Coalition Reply Br. 20).           See, e.g.,
Intercollegiate Broad., 796 F.3d at 124 (raising Appointments
Clause defense in a “subsequent proceeding” based on the
“continuing taint arising from the first” proceeding); Doolin,
139 F.3d at 212 (raising Appointments Clause challenge to
officer who issued the initial “Notice of Charges” to
collaterally attack the ultimate cease-and-desist order issued by
a validly appointed officer).
     In that scenario, Codrea reasons, the appointment issue
arose only as an affirmative defense; no act intervened during
litigation to eliminate the factual basis for an affirmative claim
for relief in a way that generally would trigger mootness
analysis. Here, by contrast, Codrea has raised as a plaintiff an
independent, pre-enforcement challenge to an agency rule in an
attempt to avert a present duty to comply, and he filed suit at a
time when the allegedly improperly appointed official was still
in office and enforcing his own challenged decision. For that
reason, the effect of Attorney General Barr’s intervening
ratification must be guided not by a merits analysis, but rather
                                19
by mootness. Codrea Br. 20–21 (adopting Coalition Reply Br.
17); see, e.g., EEOC v. First Citizens Bank of Billings, 758 F.2d
397, 399–400 (9th Cir. 1985) (treating congressional
ratification as causing mootness); see also Thomas v. Network
Solutions, Inc., 176 F.3d 500, 506 (D.C. Cir. 1999) (assuming
that congressional ratification mooted an unauthorized-tax
claim).
      The problem for Codrea is that, even if we were to adopt
his proposed analytical approach, his claim still lacks any
discernible likelihood of success on the merits because no
exception to mootness fits this scenario.
     First, this case does not implicate the exception to
mootness for cases that are “capable of repetition, yet evading
review.” United States v. Sanchez-Gomez, 138 S. Ct. 1532,
1540 (2018). For a controversy to be “capable of repetition,”
Codrea bears the burden of showing that (i) the challenged
action is “in its duration too short to be fully litigated prior to
its cessation or expiration,” and (ii) there is a “reasonable
expectation that the same complaining party will be subject to
the same action again.” Davis v. FEC, 554 U.S. 724, 735
(2008) (citations omitted); Honeywell Int’l, Inc. v. Nuclear
Regulatory Comm’n, 628 F.3d 568, 576 (D.C. Cir. 2010) (party
asserting capable of repetition bears burden of proof) (citing
Southern Co. Servs., Inc. v. FERC, 416 F.3d 39, 43 (D.C. Cir.
2005)). Under that test, “[t]he ‘wrong’ that is, or is not, ‘capable
of repetition’ must be defined in terms of the precise
controversy it spawns.” People for Ethical Treatment of
Animals, Inc. v. Gittens, 396 F.3d 416, 422–423 (D.C. Cir.
2005) (emphasis added). This demand for particularity ensures
“that courts resolve only continuing controversies between the
parties.” Id.
    Here, Codrea has wholly failed to show that appointments
claims like his are too short-fused to obtain judicial resolution,
                                 20
or that there is anything more than the most remote and
“theoretical[ ] possib[ility]” of repetition. Nelson v. Miller, 570
F.3d 868, 882 (7th Cir. 2009). For Codrea’s legal injury to
recur, (i) the Attorney General would have to leave office; (ii)
the President would then have to appoint a mere employee in
his stead (something Codrea argues has not happened more
than a “handful” of times in history (Codrea Br. 20-21
(adopting Coalition Br. 38; Coalition Reply Br. 15–16)); (iii)
that the new Acting Attorney General would then have to
promulgate a legislative rule; and (iv) by sheer coincidence,
that rule would have to adversely affect Codrea or his co-
plaintiffs’ legal rights. It takes more than such quixotic
speculation to save a case from mootness, even when the
Executive continues to defend its prerogatives in litigation. See
Larsen v. United States Navy, 525 F.3d 1, 4 (D.C. Cir. 2008).
     Second, Codrea’s invocation of the rule that a defendant’s
voluntary cessation of challenged activity will not moot a case
fares no better. See Friends of the Earth, Inc. v. Laidlaw
Environmental Servs., 528 U.S. 167, 189 (2000). The
voluntary-cessation rule is designed to deter the wrongdoer
who would otherwise “engage in unlawful conduct, stop when
sued to have the case declared moot, then pick up where he left
off, repeating this cycle until he achieves all his unlawful
ends.” Already, LLC v. Nike, Inc., 568 U.S. 85, 91 (2013). For
that reason, a party’s voluntary cessation of challenged conduct
will not moot a case unless it is “absolutely clear that the
allegedly wrongful behavior could not reasonably be expected
to recur.” Laidlaw Environmental Servs., 528 U.S. at 189
(internal quotation marks omitted).2


2
  It bears noting that the merits-based analysis of prejudice that
Codrea seeks to avoid includes a somewhat analogous exception for
a defendant’s strategic manipulation of the process to avoid judicial
                                 21
     The voluntary-cessation doctrine has no apparent
relevance here. That is because the power to effect the legally
relevant ratification by a duly installed Attorney General—the
supposed source of “cessation”—lies beyond the unilateral
legal authority of any of the named defendants, the Office of
the Attorney General, or even the President of the United
States. Under the peculiar circumstances of this case, where
the ratification was a result of the combined actions of a
presidential nomination and an independent Senate
confirmation, the “voluntariness” in “voluntary cessation” is
not implicated.
     Aimed as it is at party manipulation of the judicial process
through the false pretense of singlehandedly ending a dispute,
the voluntary-cessation exception presupposes that the
infringing party voluntarily exercises its own unilateral power
not only to terminate the suit and evade judicial review, but also
to “pick up where he left off” and complete the devious “cycle”
after the litigation is dismissed. Already, LLC, 568 U.S. at 91;
see City News & Novelty, Inc. v. City of Waukesha, 531 U.S.
278, 284 n.1 (2001) (explaining that the “rule traces to the
principle that a party should not be able to evade judicial
review, or to defeat a judgment, by temporarily altering
questionable behavior”) (emphasis added); Knox v. Service
Emps. Int’l Union, 567 U.S. 298, 307 (2012) (voluntary
cessation concerns a defendant’s “resumption of * * *
challenged conduct as soon as the case is dismissed”)


review. See Sugar Cane Growers Coop. of Fla. v. Veneman, 289
F.3d 89, 96 (D.C. Cir. 2002) (“[I]f the government could skip [the
APA’s rulemaking] procedures, engage in informal consultation, and
then be protected from judicial review unless a petitioner could show
a new argument—not [already] presented informally,” then the
APA’s prescribed rulemaking process “obviously would be
eviscerated.”).
                                22
(emphasis added); United States v. W.T. Grant Co., 345 U.S.
629, 632 (1953) (voluntary-cessation doctrine rooted in
concern over leaving a “defendant * * * free to return to his old
ways”).
     That framework ill fits a situation where, as here, the
intervening acts of independent third parties are essential to
accomplish a legally relevant change in circumstances. Here,
ratification materially changed the circumstances of the
litigation only because it was undertaken by a validly appointed
Attorney General whose authority to act Codrea does not
challenge. Codrea Br. 20–21 (adopting Coalition Reply Br. 22)
(“Plaintiff assumes that the ratification was not tainted by Mr.
Whitaker’s actions in promulgating the Rule in the first
place.”). That “cessation” of the legal challenge was outside
the hands of the named defendants—then-Acting Attorney
General Whitaker, the Bureau of Alcohol, Tobacco, Firearms
and Explosives, Acting Bureau Director Thomas Brandon, and
Attorney General William Barr. The essential predicate for
that legally relevant form of cessation was the (non-defendant)
President’s nomination and the (non-defendant) Senate’s
independent confirmation of a new Attorney General, and their
endowment of him with the authority to “cease” the litigation
by way of ratification.

     In other words, the defendants in this case lacked the
unilateral power, or the power at all, to voluntarily cease and
restart the conduct complained of—having a Reform-Act-
appointed Acting Attorney General promulgate or enforce a
rule adversely affecting Guedes and Codrea. Without such
power, the risk of manipulating the litigation process
evaporates. In addition, the deliberative burdens of the
Senate’s intervening and independent advice-and-consent role
extinguish the strategic concerns animating the voluntary-
cessation doctrine in the first place. Cf. Clarke v. United States,
915 F.2d 699, 705 (D.C. Cir. 1990) (en banc) (raising “serious
                                  23
doubts” about “applying the doctrine to Congress” because, “in
the absence of overwhelming evidence (and perhaps not then),
it would seem inappropriate for the courts either to impute such
manipulative conduct to a coordinate branch of government, or
to apply against that branch a doctrine that appears to rest on
the likelihood of a manipulative purpose”); United States Dep’t
of the Treasury v. Galioto, 477 U.S. 556, 560 (1986) (analyzing
the mooting effects of Congressional amendment without
reference to voluntary cessation). At the very least, Codrea has
a vanishingly low likelihood of prevailing on that theory.3

     In sum, because Codrea has shown no likelihood of
success on his appointment-based challenges due to Attorney
General Barr’s independent and unchallenged ratification of
the Bump-Stock Rule, the district court did not abuse its
discretion in denying a preliminary injunction based on those
statutory and constitutional claims.




3
 This case does not present, and we need not decide, whether the
President’s unilateral designation of a different acting Attorney
General would have implicated the voluntary-cessation doctrine. See
Trinity Lutheran Church of Columbia, Inc. v. Comer, 137 S. Ct.
2012, 2019 n.1 (2017) (no mootness when Governor ordered state
Department of Natural Resources to rescind challenged policy,
where there was no evidence the Department “could not revert to its
policy of excluding religious organizations”); cf. Doe v. Harris, 696
F.2d 109, 113 (D.C. Cir. 1982) (applying the capable of repetition
doctrine to “different official actors” within the same U.S. Attorney’s
Office). What matters in this case is not that the Bump-Stock Rule
was ratified by someone other than Acting Attorney General
Whitaker, but that it was ratified by someone whose authority to
undertake such a ratification—by virtue of Presidential nomination
and Senate confirmation—Codrea admits he cannot challenge.
                                24
                                B

    We next consider the plaintiffs’ contention that the Bureau
lacked statutory authority to promulgate the Bump-Stock Rule.
Specifically, Guedes and Codrea argue that the statutory
definition of “machinegun” cannot be read to include bump-
stock devices. Guedes and Codrea have not demonstrated a
substantial likelihood of success on that claim.

                                1

    At the outset, we must determine the standard by which to
assess the Rule’s conclusion that bump-stock devices amount
to “machineguns” under the statutory definition. In particular,
should we examine the Rule’s conclusion to that effect under
the Chevron framework, or is Chevron inapplicable?

      If Chevron treatment is in order, we first ask if the statute
is ambiguous concerning whether bump-stock devices can be
considered “machineguns”; and if so, we sustain the Rule’s
conclusion that bump-stock devices are machine guns as long
as it is reasonable. See, e.g., Entergy Corp. v. Riverkeeper, Inc.,
556 U.S. 208, 218 (2009). Crucially, at this second step under
Chevron, an “agency need not adopt * * * the best reading of
the statute, but merely one that is permissible.” Dada v.
Mukasey, 554 U.S. 1, 29 n.1 (2008). Conversely, if Chevron’s
two-step framework is inapplicable, we accept the agency’s
interpretation only if it is the best reading of the statute.

    Much, then, can turn on whether an agency’s interpretation
merits treatment under Chevron. For that reason, and because
none of the parties presents an argument for applying the
Chevron framework (the plaintiffs contend that Chevron is
inapplicable and the government does not argue otherwise), we
devote considerable attention to the question of Chevron’s
                               25
applicability to the Bump-Stock Rule. We conclude that the
Rule warrants consideration under Chevron.

                                a

     The applicability of Chevron materially depends on what
kind of rule the Bump-Stock Rule represents. There is a
“central distinction” under the Administrative Procedure Act
between legislative rules and interpretive rules. Chrysler Corp
v. Brown, 441 U.S. 281, 301 (1979); see 5 U.S.C. § 553(b), (d).
And that distinction centrally informs the applicability of
Chevron. “Legislative rules generally receive Chevron
deference,” Nat’l Mining Ass’n v. McCarthy, 758 F.3d 243, 251
(D.C. Cir. 2014), whereas “interpretive rules * * * enjoy no
Chevron status as a class,” United States v. Mead Corp., 533
U.S. 218, 232 (2001); see also Nat’l Mining Ass’n, 758 F.3d at
251 (observing that interpretive rules “often do not” receive
Chevron deference).

     Legislative rules result from an agency’s exercise of
“delegated legislative power” from Congress. Am. Mining
Cong. v. Mine Safety & Health Admin., 995 F.2d 1106, 1109
(D.C. Cir. 1993). Accordingly, legislative rules have the “force
and effect of law.” Encino Motorcars, LLC v. Navarro, 136 S.
Ct. 2117, 2122 (2016). Interpretive rules, on the other hand,
are “issued by an agency to advise the public of the agency’s
construction of the statutes and rules which it administers.”
Shalala v. Guernsey Mem’l Hosp., 514 U.S. 87, 99 (1995).
Because they are not an exercise of delegated legislative
authority, interpretive rules “do not have the force and effect of
law and are not accorded that weight in the adjudicatory
process.” Id. While legislative rules generally require notice
and comment, interpretive rules need not issue pursuant to any
formalized procedures. See 5 U.S.C. § 553(b).
                               26
     To determine whether a rule is legislative or interpretive,
we ask whether the agency “intended” to speak with the force
of law. Encino Motorcars, 136 S. Ct. at 2122; Am. Mining
Cong., 995 F.2d at 1109. Central to the analysis is the
“language actually used by the agency.” Cmty. Nutrition Inst.
v. Young, 818 F.2d 943, 946 (D.C. Cir. 1987) (per curiam). We
also consider “whether the agency has published the rule in the
Code of Federal Regulations” and “whether the agency has
explicitly invoked its general legislative authority.” Am.
Mining Cong., 995 F.2d at 1112.

     All pertinent indicia of agency intent confirm that the
Bump-Stock Rule is a legislative rule. The Rule unequivocally
bespeaks an effort by the Bureau to adjust the legal rights and
obligations of bump-stock owners—i.e., to act with the force
of law. The Rule makes clear throughout that possession of
bump-stock devices will become unlawful only as of the Rule’s
effective date, not before.

     To that end, the Rule informs bump-stock owners that their
devices “will be prohibited when this rule becomes effective.”
83 Fed. Reg. at 66,514 (emphasis added). It correspondingly
assures bump-stock owners that “[a]nyone currently in
possession of a bump-stock-type device is not acting
unlawfully unless they fail to relinquish or destroy their device
after the effective date of this regulation.” Id. at 66,523
(emphasis added).        And the Rule “provides specific
information about acceptable methods of disposal, as well as
the timeframe under which disposal must be accomplished to
avoid violating 18 U.S.C. § 922(o).” Id. at 66,530 (emphasis
added).     Reinforcing the point, the Rule says it will
“criminalize only future conduct, not past possession of bump-
stock-type devices that ceases by the effective date.” Id. at
66,525 (emphasis added).
                               27
    Those statements, and others like them in the Rule,
embody an effort to “directly govern[] the conduct of members
of the public, affecting individual rights and obligations.”
Long Island Care at Home, Ltd. v. Coke, 551 U.S. 158, 172
(2007) (internal quotation marks omitted). That is powerful
evidence that the Bureau “intended [the Rule] as a binding
application of its rulemaking authority.” Id.

     The Bureau further evinced its intent to exercise legislative
authority by expressly invoking the Chevron framework and
then elaborating at length as to how Chevron applies to the
Rule. The Rule observes that, “[w]hen a court is called upon
to review an agency’s construction of the statute it administers,
the court looks to the framework set forth in Chevron U.S.A.,
Inc. v. Natural Resources Defense Council, Inc.” 83 Fed. Reg.
at 66,527. The Rule then contains several paragraphs of
analysis describing the application of each of Chevron’s two
steps to the Rule. That discussion is compelling evidence that
the Bureau did not conceive of its rule as merely interpretive.
Because “interpretive rules * * * enjoy no Chevron status as a
class,” Mead, 533 U.S. at 232, the Bureau’s exegesis on
Chevron would have served no purpose unless the agency
intended the Rule to be legislative in character.

     Other evidence of agency intent points to the same
conclusion. One consideration under our decisions is “whether
the agency has explicitly invoked its general legislative
authority.” Am. Mining Cong., 995 F.2d at 1112. The Rule
does exactly that, invoking two separate delegations of
legislative authority. See 83 Fed. Reg. at 66,515. The first is
18 U.S.C. § 926(a), which empowers the Attorney General to
“prescribe only such rules and regulations as are necessary to
carry out the provisions of [the Gun Control Act].” The second
is 26 U.S.C. § 7805(a), which grants the Attorney General
authority to “prescribe all needful rules and regulations” for the
                               28
enforcement of the National Firearms Act. See 26 U.S.C.
§ 7801(a)(2)(A). Both of those provisions, the Rule states, vest
“the responsibility for administering and enforcing the NFA
and GCA” in the Attorney General. 83 Fed. Reg. at 66,515.

     The Rule’s publication in the Code of Federal Regulations
also indicates that it is a legislative rule. See Am. Mining
Cong., 995 F.2d at 1112. By statute, publication in the Code
of Federal Regulations is limited to rules “having general
applicability and legal effect.” 44 U.S.C. § 1510 (emphasis
added). The Bump-Stock Rule amends three sections of the
Code, modifying the regulatory definition of “machine gun”
and “adding a sentence to clarify that a ‘machine gun’
includes * * * a bump-stock-type device.” 83 Fed. Reg. at
66,519 (amending 27 C.F.R. §§ 447.11, 478.11, 479.11).
Those sorts of amendments would be highly unusual for a mere
interpretive rule.

     In short, the Rule confirms throughout, in numerous ways,
that it intends to speak with the force of law. It contained all
of those indicia uniformly conveying its intended legislative
character when Acting Attorney General Whitaker issued it.
And it still contained those indicia when Attorney General Barr
subsequently ratified it.

     Notwithstanding all of that, the government’s litigating
position in this case seeks to reimagine the Rule as merely
interpretive. The government’s briefing says that the Rule is
“not an act of legislative rulemaking,” and that the Rule instead
only “sets forth the agency’s interpretation of the best reading
of the statutory definition of ‘machinegun.’” Gov’t Br. 38.

    The government’s position to that effect has highly
significant implications for owners of bump-stock devices.
Whereas a legislative rule, as an exercise of delegated
                               29
lawmaking authority, can establish a new legal rule going
forward, an interpretive rule by nature simply communicates
the agency’s interpretation of what a statute has always meant.
So here, if the Bump-Stock Rule is merely interpretive, it
conveys the government’s understanding that bump-stock
devices have always been machine guns under the statute. The
government says exactly that in its brief, observing that, per the
interpretation set out in the Rule, “any bump stock made after
1986 has always been a machinegun.” Gov’t Br. 38.

     That in turn would mean that bump-stock owners have
been committing a felony for the entire time they have
possessed the devices. Under 18 U.S.C. § 922(o)(1), it is
“unlawful for any person to transfer or possess a machinegun,”
and violators “shall be fined [or] imprisoned not more than 10
years, or both,” id. § 924(a)(2).         As the government
acknowledges, under the view it espouses in its brief that the
Rule is interpretive, the possession of bump stocks “has always
been banned.” Gov’t Br. 38. And that would be so
notwithstanding a number of prior contrary interpretations by
the agency. See 83 Fed. Reg. at 13,444–13,446.

     The government’s account of the Rule in its brief—
including its position that bump-stock owners have always
been felons—is incompatible with the Rule’s terms. The Rule
gives no indication that bump stocks have always been
machine guns or that bump-stock owners have been
committing a felony for the entire time they have possessed the
device. The Rule in fact says the opposite. After all, it
establishes an effective date, after which (and only after which)
bump-stock possession will be prohibited. 83 Fed. Reg. at
66,523. A future effective date of that kind cannot be
reconciled with a supposed intent to convey that bump-stock
possession “has always been banned.” Gov’t Br. 38.
                                30
     The government now characterizes the Rule’s effective
date as merely marking the end of a period of discretionary
withholding of enforcement, in that the Rule informs the public
that the Department will “not pursue enforcement action
against individuals who sold or possessed bump stocks prior to
the effective date.” Id. at 38–39. Once again, that is not what
the Rule says. The government engages in enforcement
discretion when it voluntarily refrains from prosecuting a
person even though he is acting unlawfully. The Rule, by
contrast, announces that a person “in possession of a bump-
stock type device is not acting unlawfully unless they fail to
relinquish or destroy their device after the effective date of this
regulation.” 83 Fed. Reg. at 66,523 (emphases added). That is
the language of a legislative rule establishing when bump-stock
possession will become unlawful, not an interpretive rule
indicating it has always been unlawful.

     In short, the government cannot now, in litigation,
reconceive the Bump-Stock Rule as an interpretive rule. The
character of a rule depends on the agency’s intent when issuing
it, not on counsel’s description of the rule during subsequent
litigation. See Encino Motorcars, 136 S. Ct. at 2122; cf. SEC
v. Chenery Corp., 318 U.S. 80, 87–88 (1943). Here, that intent
is unmistakable: the Bump-Stock rule is a legislative rule.

                                b

     Ordinarily, legislative rules receive Chevron deference.
See Nat’l Mining Ass’n, 758 F.3d at 251. This legislative rule
is no different.

    The Supreme Court has established that we afford
Chevron deference if we determine (i) “that Congress
delegated authority to the agency generally to make rules
carrying the force of law,” and (ii) “that the agency
                               31
interpretation claiming deference was promulgated in the
exercise of that authority.” Mead, 533 U.S. at 226–227 (2001).
Here, both are true.

     First, we know Congress intended a delegation of
legislative authority to the agency because Congress made the
relevant delegations express. As noted, the Attorney General
has the power to prescribe “such rules and regulations as are
necessary to carry out the provisions of” the Gun Control Act.
18 U.S.C. § 926(a). And the Attorney General “shall prescribe
all needful rules and regulations for the enforcement of” the
National Firearms Act.         26 U.S.C. § 7805(a); see id.
§ 7801(a)(2)(A).      “[A] general conferral of rulemaking
authority” of that variety “validate[s] rules for all the matters
the agency is charged with administering.” City of Arlington v.
FCC, 569 U.S. 290, 306 (2013). The Supreme Court has said
exactly that for § 7805(a), one of the delegations of authority
at issue. Specifically discussing that very provision, the Court
explained that it has “found such ‘express congressional
authorizations to engage in the process of rulemaking’ to be ‘a
very good indicator of delegation meriting Chevron
treatment.’” Mayo Found. for Med. Educ. & Research v.
United States, 562 U.S. 44, 57 (2011) (quoting Mead, 533 U.S.
at 229).

    Second, we know that the Bureau promulgated the Bump-
Stock Rule “in the exercise of that authority” to “make rules
carrying the force of law” because that criterion is the defining
characteristic of a legislative rule. Mead, 533 U.S. at 227. And
we have already determined that the Rule is legislative in
character. We are then firmly within Chevron’s domain.

   Nonetheless, the parties protest the applicability of
Chevron on several grounds. The plaintiffs first argue that
Chevron deference has been waived or forfeited by the
                                32
government. Next, the parties (including the government)
submit that Chevron deference is inapplicable in the context of
criminal statutes. And finally, Guedes contends that Chevron
deference for criminal statutes is displaced by the rule of lenity.
None of those objections to applying Chevron, we conclude, is
likely to succeed in the context of the Bump-Stock Rule.

                                (i)

     The agency plainly believed it was acting in a manner
warranting Chevron treatment given that it expressly invoked
the Chevron framework in the Rule. The plaintiffs assert that
the government nonetheless has forfeited, or even waived, the
application of Chevron deference by declining to argue for it in
this litigation. And while the government has not taken a
definitive position before us on whether Chevron can be
waived or forfeited, it has declined to invoke Chevron
throughout the course of the litigation.

      In particular, in its briefing before the district court, the
government expressly disclaimed any entitlement to Chevron
deference. And after the district court nonetheless relied on
Chevron to affirm the Rule, the government filed notices in
other pending challenges to the Rule, stating that it “ha[s] not
contended that the deference afforded under Chevron * * *
applies in this action.’” E.g., Notice of Supplemental Authority
at 2, Gun Owners of Am., Inc. v. Barr, No. 1:18-cv-1429 (W.D.
Mich. Feb. 27, 2019), ECF No. 38. Now, in this appeal, the
government affirmatively disclaims any reliance on Chevron.
See Gov’t Br. 37. And at oral argument, the government went
so far as to indicate that, while it believes the Rule should be
upheld as the best reading of the statute without any need for
Chevron deference, if the Rule’s validity turns on the
applicability of Chevron, it would prefer that the Rule be set
                               33
aside rather than upheld under Chevron. Oral Argument at
42:38–43:45.

     To the extent Chevron treatment can be waived, we
assume that the government’s posture in this litigation would
amount to a waiver rather than only a forfeiture. See Wood v.
Milyard, 566 U.S. 463, 470 n.4 (2012) (“A waived claim or
defense is one that a party has knowingly and intelligently
relinquished; a forfeited plea is one that a party has merely
failed to preserve.”). But our court has yet to address whether,
when an agency promulgates a rule that would otherwise
plainly occasion the application of Chevron, agency counsel
could nonetheless opt to effect a waiver of Chevron treatment
when later defending against a challenge to the rule.

     We have, however, held that an agency’s lawyers cannot
forfeit the applicability of Chevron deference unless the
underlying agency action fails to “manifests its engagement in
the kind of interpretive exercise to which review under
Chevron generally applies—i.e., interpreting a statute it is
charged with administering in a manner (and through a
process) evincing an exercise of its lawmaking authority.”
SoundExchange, Inc. v. Copyright Royalty Bd., 904 F.3d 41, 54
(D.C. Cir. 2018). We grounded our holding in the principle
that “it is the expertise of the agency, not its lawyers,” that
underpins Chevron. Id. (quoting Peter Pan Bus Lines, Inc. v.
Fed. Motor Carrier Safety Admin., 471 F.3d 1350, 1354 n.3
(D.C. Cir. 2006)); see also Chenery, 318 U.S. at 87–88. We
see no reason that the same limitations on forfeiture of Chevron
should not also govern waiver of Chevron.

    Forfeiture and waiver involve, respectively, a failure to
invoke, or an affirmative decision not to invoke, a party’s “right
or privilege.” Johnson v. Zerbst, 304 U.S. 458, 464 (1938).
But Chevron is not a “right” or “privilege” belonging to a
                                34
litigant. It is instead a doctrine about statutory meaning—
specifically, about how courts should construe a statute.

    If a statute contains ambiguity, Chevron directs courts to
construe the ambiguity as “an implicit delegation from
Congress to the agency to fill in the statutory gaps.” FDA v.
Brown & Williamson Tobacco Corp., 529 U.S. 120, 159
(2000). If there is ambiguity, the meaning of the statute
becomes whatever the agency decides to fill the gaps with, as
long as the agency’s interpretation is reasonable and “speak[s]
with the force of law.” Mead, 533 U.S. at 229. And insofar as
Chevron concerns the meaning of a statute, it is an awkward
conceptual fit for the doctrines of forfeiture and waiver.

     We, for example, would give no mind to a litigant’s failure
to invoke interpretive canons such as expressio unius or
constitutional avoidance even if she intentionally left them out
of her brief. “[T]he court is not limited to the particular legal
theories advanced by the parties, but rather retains the
independent power to identify and apply the proper
construction of governing law.” Kamen v. Kemper Fin. Servs.,
Inc., 500 U.S. 90, 99 (1991). The “independent power” to
identify and apply the correct law presumably includes
application of the Chevron framework when determining the
meaning of a statute.

     Allowing an agency to freely waive Chevron treatment in
litigation also would stand considerably in tension with basic
precepts of administrative law. As we have explained, a
legislative rule qualifying for Chevron deference remains
legislative in character even if the agency claims during
litigation that the rule is interpretive: Chenery instructs that the
proper subject of our review is what the agency actually did,
not what the agency’s lawyers later say the agency did. See
318 U.S. at 87–88. Accordingly, we have held that a particular
                                35
rule is legislative rather than interpretive over the protestations
of the agency. See, e.g., Cmty. Nutrition Inst., 818 F.2d at 946.
And once we conclude that a rule is legislative, it follows that
we generally review the rule’s validity under the Chevron
framework. See Nat’l Mining Ass’n, 758 F.3d at 251.

      A waiver regime, moreover, would allow an agency to
vary the binding nature of a legislative rule merely by asserting
in litigation that the rule does not carry the force of law, even
though the rule speaks to the public with all the indicia of a
legislative rule. Agency litigants then could effectively amend
or withdraw the legal force of a rule without undergoing a new
notice-and-comment rulemaking. That result would enable
agencies to circumvent the Administrative Procedure Act’s
requirement “that agencies use the same procedures when they
amend or repeal a rule as they used to issue the rule in the first
instance.” Perez v. Mortg. Bankers Ass’n, 135 S. Ct. 1199,
1206 (2015). And an agency could attempt to secure rescission
of a policy it no longer favors without complying with the
Administrative Procedure Act, or perhaps could avoid the
political accountability that would attend its own policy
reversal by effectively inviting the courts to set aside the rule
instead.

     We thus conclude, consistent with SoundExchange’s
approach to forfeiture of Chevron, that an agency’s lawyers
similarly cannot waive Chevron if the underlying agency action
“manifests its engagement in the kind of interpretive exercise
to which review under Chevron generally applies.”
SoundExchange, 904 F.3d at 54. In that event, we “apply
Chevron * * * even if there is no invocation of Chevron in the
briefing in our court.” Id.

    In this case, the Bump-Stock Rule plainly indicates the
agency’s view that it was engaging in a rulemaking entitled to
                               36
Chevron deference. That observation naturally follows from
the Rule’s legislative character, which generally yields
treatment under Chevron. See Nat’l Mining Ass’n, 758 F.3d at
251. And for this Rule in particular, another telltale sign of the
agency’s belief that it was promulgating a rule entitled to
Chevron deference is the Rule’s invocation of Chevron by
name. To be sure, an agency of course need not expressly
invoke the Chevron framework to obtain Chevron deference:
“Chevron is a standard of judicial review, not of agency
action.” SoundExchange, 904 F.3d at 54. Still, the Bureau’s
invocation of Chevron here is powerful evidence of its intent
to engage in an exercise of interpretive authority warranting
Chevron treatment.

     The Bureau, in rejecting objections that the agency’s
interpretation “would not be entitled to deference under
Chevron,” 83 Fed. Reg. at 66,526, specifically invoked the
Chevron framework and marched through its two-step
analysis, id. at 66,527. At step one, the agency explained that
its interpretation “accord[ed] with the plain meaning” of the
statute. And at step two, the agency explained that it “ha[d] the
authority to interpret elements of the definition of
‘machinegun’ like ‘automatically’ and ‘single function of the
trigger,’” concluding that its “construction of those terms is
reasonable under Chevron [Step Two].” Id.

     The Rule expressly defends the agency’s reading of the
statute as an interpretive exercise implicating Chevron.
Agency counsel’s later litigating decision to refrain from
invoking Chevron thus affords no basis for our denying the
Rule Chevron status.
                               37
                               (ii)

     Next, the plaintiffs submit that Chevron deference has no
application to regulations interpreting statutes like the National
Firearms Act and the Gun Control Act because they impose
criminal penalties on violators. Chevron deference in the
context of such statutes, the plaintiffs urge, would flout an
understanding that “criminal laws are for courts, not for the
Government, to construe.” Abramski v. United States, 573 U.S.
169, 191 (2014). And the plaintiffs are not the only parties who
question Chevron’s salience in the criminal context. The
government’s decision to refrain from invoking Chevron in this
litigation appears to stem from the same concerns. See Gov’t
Br. 36–37.

     Guedes and Codrea, however, have failed to demonstrate
a likelihood of success in establishing a general rule against
applying Chevron to agency interpretations of statutes that
have criminal-law implications. To the contrary, precedent
says otherwise.

     Start with Chevron itself. At issue in Chevron was the
meaning of the term “stationary source” in the Clean Air Act.
See Chevron, 467 U.S. at 840. The scope of that term defined
the statutory obligation of private parties, under state
implementation plans, to obtain permits for the construction
and operation of “new or modified major stationary sources of
air pollution.” 42 U.S.C. § 7502(a)(1), (b)(6) (1982). But at
the time, any person who knowingly violated any requirement
of a state implementation plan (after notice from the EPA)
faced a fine of $25,000 a day or imprisonment for up to a year,
or both. See id. § 7413(c)(1) (1982). Nevertheless, the
Chevron Court established the decision’s namesake deference.
                               38
     For another example, consider the securities laws. The
SEC’s interpretation of those laws regularly receives Chevron
treatment, e.g., Nat’l Ass’n of Mfrs. v. SEC, 748 F.3d 359, 366
(D.C. Cir. 2014); Am. Equity Inv. Life Ins. Co. v. SEC, 613 F.3d
166, 172–173 (D.C. Cir. 2010); Markowski v. SEC, 274 F.3d
525, 528–529 (D.C. Cir. 2001), even though their violation
often triggers criminal liability. The Securities Exchange Act,
for instance, imposes criminal sanctions for willful violations
of “any provision” of the Act or “any rule or regulation
thereunder the violation of which is made unlawful.” 15 U.S.C.
§ 78ff(a). Yet in United States v. O’Hagan—a criminal case—
the Supreme Court accorded Chevron deference to an SEC rule
that interpreted a provision of the Act in a manner rendering
the defendant’s conduct a crime. 521 U.S. 642, 667, 673
(1997) (citing Chevron, 467 U.S. at 844). The Court noted that
Congress had authorized the Commission “to prescribe
legislative rules,” and held that the rule in question, issued in
an exercise of that authority, should receive “controlling
weight” under Chevron. Id. at 673 (quoting Chevron, 467 U.S.
at 844).

     While the Court in O’Hagan applied Chevron in a criminal
case, it (like Chevron itself) did not specifically address
whether the criminal context should have afforded a basis for
denying deference to the agency’s interpretation. But the Court
engaged with that precise issue in Babbitt v. Sweet Home
Chapter of Communities for a Great Oregon, 515 U.S. 687
(1995). There, the Court reviewed a regulation interpreting the
term “take” in the Endangered Species Act. The challengers
argued that Chevron deference was inappropriate because the
Endangered Species Act included criminal penalties for certain
violations. See id. at 704 n.18. The Court disagreed, holding
that, notwithstanding the statute’s criminal penalties, it would
defer “to the Secretary’s reasonable interpretation” under
Chevron. See id. at 703–704 & 704 n.18.
                                39
     Our circuit precedent is in accord.           Recently, in
Competitive Enterprise Institute v. United States Department
of Transportation, 863 F.3d 911 (D.C. Cir. 2017), we explained
that “[w]e apply the Chevron framework * * * even though
violating [the statute] can bring criminal penalties,” id. at 915
n.4 (citing Babbitt, 515 U.S. at 704 n.18); see id. at 921
(Kavanaugh, J., concurring) (“I join the majority opinion[.]”).
That precedent is controlling here. See also Humane Society v.
Zinke, 865 F.3d 585, 591, 595 (D.C. Cir. 2017) (applying
Chevron even though the challenged rule interpreted the
Endangered Species Act, the violation of which results in
“criminal sanctions”).

     Also, at least twice before, we afforded Chevron deference
to an agency’s construction of a statute in the criminal context
over the express objection of a defendant. In United States v.
Kanchanalak, 192 F.3d 1037 (D.C. Cir. 1999), the defendants
“argue[d] that this court should not give Chevron deference to
the FEC’s interpretation of an ambiguous statute in a criminal
proceeding,” id. at 1047 n.17. We disagreed: “That criminal
liability is at issue does not alter the fact that reasonable
interpretations of the act are entitled to deference.” Id. (citing
Babbitt, 515 U.S. at 703–705). And in In re Sealed Case, 223
F.3d 775 (D.C. Cir. 2000), we again declined to forgo Chevron
in a criminal context, holding that “[d]eference is due as much
in a criminal context as in any other,” id. at 779 (citing Babbitt,
515 U.S. at 703–705).

     To be sure, the Supreme Court has signaled some wariness
about deferring to the government’s interpretations of criminal
statutes. See Abramski, 573 U.S. at 191; see also United States
v. Apel, 571 U.S. 359, 369 (2014) (“[W]e have never held that
the Government's reading of a criminal statute is entitled to any
deference.”). But those statements were made outside the
context of a Chevron-eligible interpretation—that is, outside
                               40
the context of an agency “speak[ing] with the force of law.”
Mead, 533 U.S. at 229. In Abramski, the Court declined to
extend deference to informal guidance documents published by
the Bureau. See 573 U.S. at 191. And in Apel, the Court
declined to defer to an interpretation contained in “Executive
Branch documents” that were “not intended to be binding.”
571 U.S. at 368. When directly faced with the question of
Chevron’s applicability to an agency’s interpretation of a
statute with criminal applications through a full-dress
regulation, the Court adhered to Chevron. See Babbitt, 515
U.S. at 704 n.18.

     That holding, and our court’s precedents, govern us here
and call for the application of Chevron. The parties have
identified no distinction between the provision at issue in this
case and the provisions with criminal penalties to which
Chevron deference has been applied. The briefing contains
nary a word suggesting any distinction between this case and
prior decisions applying Chevron in criminal contexts. And
neither Guedes nor counsel for the government offered any
distinction even when specifically asked at oral argument. See
Oral Argument at 6:08–7:15, 45:45–49:00.

     Nothing in the relevant statutory delegations of authority,
moreover, suggests a basis for denying Chevron treatment for
agency actions with criminal implications The Supreme Court
has instructed that the inquiry turns on whether the “language
of the delegation provision” is sufficiently “broad” such that it
is “clear * * * the statute gives [the] agency * * * power to
enforce all provisions of the statute.” Gonzales v. Oregon, 546
U.S. 243, 258 (2006) (emphasis added). In Gonzales, for
example, the Court found that the Attorney General lacked
power to interpret a particular criminal provision of the
Controlled Substances Act because the delegation of
rulemaking authority was too narrow and “did not delegate to
                               41
the Attorney General authority to carry out or effect all
provisions of the CSA.” Id. at 259 (emphasis added). By
contrast, the two pertinent delegation provisions in this case are
framed in broad terms. See 18 U.S.C. § 926(a) (delegating to
Attorney General the power to prescribe “such rules and
regulations as are necessary to carry out the provisions of [the
Gun Control Act]”); 26 U.S.C. § 7805(a) (delegating to
Attorney General, see id. § 7801(a)(2)(A), the power to
“prescribe all needful rules and regulations for the enforcement
of [the National Firearms Act]”).

     The statutory context bolsters the inference that Congress
intended those delegations to encompass regulations with
criminal implications. The Gun Control Act, found at Chapter
44 of Title 18, is a purely criminal statute. See 18 U.S.C.
§ 924(a)(2). Yet § 926(a) expressly delegates to the Attorney
General the power to promulgate “such rules and regulations
as are necessary to carry out the provisions of th[at] chapter.”
Similarly, the National Firearms Act, found at Chapter 53 of
Title 26, has criminal applications. See 18 U.S.C. § 924(a)(2);
United States v. Thompson/Center Arms Co., 504 U.S. 505, 517
(1992). The penalty for “fail[ing] to comply with any provision
of th[at] chapter” is a fine of up to $10,000, or imprisonment
for up to 10 years, or both. 26 U.S.C. § 5871. And yet
§ 7801(a)(2)(A) tasks the Attorney General with “[t]he
administration and enforcement of * * * Chapter 53,” including
“prescrib[ing] all needful rules and regulations for * * *
enforcement.” Id. § 7805(a).

     The plaintiffs rely on United States v. Thompson/Center
Arms Co., in which the Supreme Court applied the rule of lenity
to an ambiguous provision of the National Firearms Act. 504
U.S. at 517–518. But Babbitt later made clear that the Court in
Thompson/Center had no occasion to apply Chevron:
Thompson/Center, the Babbitt Court explained, “rais[ed] a
                              42
narrow question concerning the application of a statute that
contain[ed] criminal sanctions * * * where no regulation was
present.” Babbitt, 515 U.S. at 704 n.18 (emphasis added). If
anything, then, Babbitt implies that Chevron should apply in a
case—like this one—involving an interpretation of the
National Firearms Act where a regulation is present.

     The plaintiffs also cite United States v. McGoff, 831 F.2d
1071 (D.C. Cir. 1987), a pre-Babbitt decision that interpreted
the statute-of-limitations provision of the Foreign Agents
Registration Act. We observed in passing that, “[n]eedless to
say, in this criminal context, we owe no deference to the
Government’s interpretation of the statute.” Id. at 1080 n.17.
As in Thompson/Center, however, the McGoff Court had no
occasion to apply Chevron because the government never
asserted reliance on a regulation or other Chevron-eligible
instrument. See id.

     At oral argument, the plaintiffs suggested that permitting
an agency’s interpretation to carry the force of law in the
criminal context would infringe the separation of powers. See
Oral Argument 6:51–6:58. That suggestion is difficult to
square with the Supreme Court’s decision in Touby v. United
States, 500 U.S. 160 (1991). There, the Court upheld a
delegation of legislative authority to the Attorney General to
schedule substances under the Controlled Substances Act
against a challenge under the nondelegation doctrine. Id. at
164. The Court held that, in the criminal context, as in all
contexts, the separation of powers “does not prevent Congress
from seeking assistance * * * from its coordinate Branches” so
long as Congress “lays down by legislative act an intelligible
principle to which the person or body authorized to act is
directed to conform.” Id. at 165 (alterations omitted) (quoting
J.W. Hampton, Jr., & Co. v. United States, 276 U.S. 394, 409
                                43
(1928)). And no party suggests that such an intelligible
principle is lacking in this case.

     In short, Congress delegated authority to administer the
National Firearms Act and the Gun Control Act to the Attorney
General, and the Attorney General promulgated a legislative
rule in the exercise of that authority. Under binding precedent,
Guedes and Codrea have failed to demonstrate a likelihood of
success on their claim that the Rule is invalid just because of
its criminal-law implications.

                               (iii)

     Relatedly, Guedes argue that Chevron is inapplicable
because a different canon of interpretation, the rule of lenity,
should control instead. Under the rule of lenity, “ambiguity
concerning the ambit of criminal statutes should be resolved in
favor of lenity.” Rewis v. United States, 401 U.S. 808, 812
(1971). Guedes reasons that because Chevron is premised on
the existence of statutory ambiguity, and because the rule of
lenity resolves ambiguity in favor of the defendant, there is no
remaining ambiguity to which Chevron can apply.

     It is true that the rule of lenity generally applies to the
interpretation of the National Firearms Act and the Gun
Control Act. But in circumstances in which both Chevron and
the rule of lenity are applicable, the Supreme Court has never
indicated that the rule of lenity applies first. In fact, the Court
has held to the contrary. In Babbitt, the Court squarely rejected
the argument that “the rule of lenity should foreclose any
deference to the Secretary’s interpretation of the ESA because
the statute includes criminal penalties.” 515 U.S. at 704 n.18.
The Court observed that it had “never suggested that the rule
of lenity should provide the standard for reviewing facial
challenges to administrative regulations whenever the
                                44
governing statute authorizes criminal enforcement.” Id. The
Court proceeded to apply Chevron deference. Id. at 703.

     Our precedent takes the same tack. In Kanchanalak, we
expressly rebuffed the argument that Guedes now presses: “To
argue, as defendants do, that the rule of lenity compels us to
reject the FEC’s otherwise reasonable interpretation of an
ambiguous statutory provision [under Chevron] is to ignore
established principles of law.” 192 F.3d at 1050 n.23 (citing
Babbitt, 515 U.S. at 704 n.18).

     Those precedents are in line with the Supreme Court’s
characterization of the rule of lenity as a canon of “last resort.”
The Court has instructed that “[t]he rule comes into operation
at the end of the process of construing what Congress has
expressed, not at the beginning as an overriding consideration
of being lenient to wrongdoers.” Callanan v. United States,
364 U.S. 587, 596 (1961). Accordingly, the rule of lenity
applies only “when the ordinary canons of statutory
construction have revealed no satisfactory construction.”
Lockhart v. United States, 136 S. Ct. 958, 968 (2016). And
Chevron is a rule of statutory construction, insofar as it is a
doctrine that “constru[es] what Congress has expressed.”
Callanan, 364 U.S. at 596.

     Finally, our approach coheres with the rule of lenity’s
purposes. The doctrine serves to ensure that “legislatures and
not courts [are] defin[ing] criminal activity” and to secure “fair
warning” about the content of criminal law. United States v.
Bass, 404 U.S. 336, 348 (1971) (internal quotation marks
omitted). Chevron deference vindicates both purposes.

     First, Chevron is consistent with the separation of powers,
including for regulations defining criminal activity, because
delegations of legislative authority in the criminal sphere are
                               45
constitutional. See Touby, 500 U.S. at 165. The parties would
have us disregard Congress’s textual delegations to the agency
and do the interpretive work instead. That course, though,
would not respect the notion that “legislatures and not courts”
should take the lead. Bass, 404 U.S. at 348.

     Second, Chevron promotes fair notice about the content of
criminal law. It applies only when, at Congress’s direction,
agencies have followed “relatively formal administrative
procedure tending to foster the fairness and deliberation that
should underlie a pronouncement of such force.” Mead, 533
U.S. at 230. Importantly, such procedures, which generally
include formal public notice and publication in the Federal
Register, do not “provide such inadequate notice of potential
liability as to offend the rule of lenity.” Babbitt, 515 U.S. at
704 n.18. Tellingly, there is no suggestion of inadequate notice
here. Rather, if the Rule is a valid legislative rule, all are on
notice of what is prohibited.

     For substantially the same reasons, plaintiffs’ challenge
under the Due Process Clause cannot succeed. To apply
Chevron, Codrea notes, we must first determine that the statute
is ambiguous, but that, in Codrea’s view, would imply that the
statute is facially void for vagueness. Codrea’s challenge is
misconceived. A criminal statute is void for vagueness if it
fails to provide ordinary people “fair notice” of the conduct it
proscribes. Sessions v. Dimaya, 138 S. Ct. 1204, 1223 (2018).
But the promulgation of the Bump-Stock Rule through notice-
and-comment procedures afforded “fair notice” of the
prohibited conduct.

                               2

    Having concluded that the Chevron framework is
applicable, we now proceed to examine the Bump-Stock Rule
                                46
under it. We first ask whether the agency-administered statute
is ambiguous on the “precise question at issue.” Chevron, 467
U.S. at 842. If the statute’s meaning is unambiguous, then we
need go no further. But if we find ambiguity, we proceed to
the second step and ask whether the agency has provided a
“permissible construction” of the statute. Id. at 843. At that
stage, “the task that confronts us is to decide, not whether [the
agency’s interpretation is] the best interpretation of the statute,
but whether it represents a reasonable one.” Atl. Mut. Ins. Co.
v. Comm’r, 523 U.S. 382, 389 (1998).

     The National Firearms Act and the Gun Control Act both
define “machinegun” to mean “any weapon which shoots, is
designed to shoot, or can be readily restored to shoot,
automatically more than one shot, without manual reloading,
by a single function of the trigger.” 26 U.S.C. § 5845(b); see
18 U.S.C. § 921(a)(23). The definition of “machinegun” also
includes “any part designed and intended solely and
exclusively, or combination of parts designed and intended, for
use in converting a weapon into a machinegun, and any
combination of parts from which a machinegun can be
assembled if such parts are in the possession or under the
control of a person.” 26 U.S.C. § 5845(b).

     The Bump-Stock Rule determines that semiautomatic
rifles equipped with bump-stock-type devices are
“machineguns” because they “function[] as the result of a self-
acting or self-regulating mechanism that allows the firing of
multiple rounds” through “a single pull of the trigger.” 83 Fed.
Reg. at 66,553. Applying Chevron, we determine that the
statutory definition of “machinegun” is ambiguous and the
Bureau’s interpretation is reasonable. The plaintiffs therefore
are unlikely to succeed on the merits of their claim that the Rule
is out of step with the statutory definition.
                               47
                               a

    At Chevron’s first step, two features of the statutory
definition of “machinegun” render it ambiguous. The first is
the phase “single function of the trigger.” The second is the
word “automatically.” We discuss them in that order.

                               (i)

     As the district court recognized, the statutory phrase
“single function of the trigger” admits of more than one
interpretation. It could mean “a mechanical act of the trigger.”
Guedes, 356 F. Supp. 3d at 130. Or it could mean “a single
pull of the trigger from the perspective of the shooter.” Id.

      The first interpretation would tend to exclude bump-stock
devices: while a semiautomatic rifle outfitted with a bump
stock enables a continuous, high-speed rate of fire, it does so
by engendering a rapid bumping of the trigger against the
shooter’s stationary finger, such that each bullet is fired
because of a distinct mechanical act of the trigger. The second
interpretation would tend to include bump-stock devices: the
shooter engages in a single pull of the trigger with her trigger
finger, and that action, via the operation of the bump stock,
yields a continuous stream of fire as long she keeps her finger
stationary and does not release it. See 83 Fed. Reg. at 66,519.

     Neither of those interpretations is compelled (or
foreclosed) by the term “function” in “single function of the
trigger.” The word “function” focuses our attention on the
“mode of action,” 4 Oxford English Dictionary 602 (1933), or
“natural * * * action,” Webster’s New International Dictionary
876 (1933), by which the trigger operates. But the text is silent
on the crucial question of which perspective is relevant.
                               48
     A mechanical perspective, for instance, might focus on the
trigger’s release of the hammer, which causes the release of a
round. From that perspective, a “single function of the trigger”
yields a single round of fire when a bump-stock device moves
the trigger back and forth. By contrast, from the perspective of
the shooter’s action, the function of pulling the trigger a single
time results in repeated shots when a bump-stock device is
engaged. From that perspective, then, a “single function of the
trigger” yields multiple rounds of fire.

     In light of those competing, available interpretations, the
statute contains a “gap for the agency to fill.” Chevron, 467
U.S. at 843.

      Guedes argues that the phrase “single function of the
trigger” unambiguously compels a focus on the trigger’s
mechanical operation. He contends, for example, that
“[r]egardless of the mechanism by which the shooter acts * * *
it is the movement of the trigger releasing the hammer * * *
that define[s] the boundaries of two distinct ‘single’ functions
of the trigger.” Guedes Br. 12–13. That argument begs the
crucial question of perspective. It may be reasonable to take
the view, as Guedes does, that the mechanical operation of the
trigger is the lens through which to view its function. But to
establish a likelihood of success on the merits, Guedes and
Codrea would have to establish that reading the statute to mean
a “single pull of the trigger” by the shooter is impermissible.
They have not done so.

     At Chevron’s first step, we do not ask which of those
interpretations is the better reading of the statute. Rather, we
ask whether either of those interpretations is unambiguously
“compel[led]” by the statute, to the exclusion of the other one.
Chevron, 467 U.S. at 860. Here, we think the answer is no.
                                49
     Nor does Staples v. United States, 511 U.S. 600 (1994),
compel a particular interpretation of “single function of the
trigger.” There, in a footnote, the Court observed that a weapon
is “automatic” if it “fires repeatedly with a single pull of the
trigger”—“[t]hat is, [if] once its trigger is depressed, the
weapon will automatically continue to fire until its trigger is
released or the ammunition is exhausted.” Id. at 602 n.1. The
Court’s description, then, speaks both in terms of a “single pull
of the trigger” and a “release[]” of the trigger, id. (emphasis
added), which ultimately sheds limited light on the choice
between the two competing understandings of “function of the
trigger” that are at issue here. Regardless, the precise definition
of “single function of the trigger” was not at issue in Staples.
See id. at 602. And the Court did not purport to exclude any
interpretation as foreclosed by the statute. Cf. Nat’l Cable &
Telecomms. Ass’n v. Brand X Internet Servs., 545 U.S. 967,
996 (2005).

                               (ii)

    Similarly, the statutory term “automatically” admits of
multiple interpretations. The statute speaks in terms of a
“weapon which shoots * * * automatically more than one shot,
without manual reloading, by a single function of the trigger.”
26 U.S.C. § 5845(b); see 18 U.S.C. § 921(a)(23). The term
“automatically” does not require that there be no human
involvement to give rise to “more than one shot.” Rather, the
term can be read to require only that there be limited human
involvement to bring about more than one shot. See, e.g.,
Webster's New International Dictionary 157 (defining
“automatically” as the adverbial form of “automatic”); id. at
156 (defining “automatic” as “self-acting or self-regulating,”
especially applied to “machinery or devices which perform
parts of the work formerly or usually done by hand” (emphasis
                                50
added)). But how much human input in the “self-acting or self-
regulating” mechanism is too much?

      The plaintiffs would read the phrase “by a single function
of the trigger” to provide “the starting and the ending point of
just how much human input is allowable.” Codrea Br. 14. In
their view, then, a gun cannot be said to fire “automatically” if
it requires both a single pull of the trigger and constant pressure
on the gun’s barrel, as a bump-stock device requires. We are
unpersuaded. After all, a quite common feature of weapons
that indisputably qualify as machine guns is that they require
both a single pull of the trigger and the application of constant
and continuing pressure on the trigger after it is pulled. We
know, therefore, that the requirement of some measure of
additional human input does not render a weapon
nonautomatic. To purloin an example from the district court:
an “automatic” sewing machine still “requires the user to press
a pedal and direct the fabric.” Guedes, 356 F. Supp. 3d at 131
(emphasis added).

     That workaday example illustrates another, perhaps more
natural, reading of “automatically”: the “automatic[]”
mechanism need only be “set in motion” by a single function
of the trigger. United States v. Olofson, 563 F.3d 652, 658 (7th
Cir. 2009) (emphasis added); see also United States v. Evans,
978 F.2d 1112, n.2 (9th Cir. 1992) (“‘[B]y a single function of
the trigger’ describes the action that enables the weapon to
‘shoot automatically without manual reloading, not the
‘trigger’ mechanism.” (ellipses omitted)). That is, rather than
reading the phrase “by a single function of the trigger” to mean
“by only a single function of the trigger,” the phrase can
naturally be read to establish only the preconditions for setting
off the “automatic” mechanism, without foreclosing some
further degree of manual input such as the constant forward
pressure needed to engage the bump stock in the first instance.
                               51
And if so, then the identified ambiguity endures. How much
further input is permitted in the mechanism set in motion by
the trigger? The statute does not say.

    In sum, the statutory definition of “machinegun” contains
two central ambiguities, both of which the agency has
attempted to construe. We therefore proceed to Chevron’s
second step.

                               b

     At the second step, “the question for the court is whether
the agency’s [construction] is based on a permissible
construction of the statute.” Chevron, 467 U.S. at 843. Guedes
and Codrea are not likely to succeed in showing that the agency
has impermissibly interpreted both ambiguities.

     The Bureau’s interpretation of “single function of the
trigger” to mean “single pull of the trigger” is a permissible
reading of the statute. The Bureau is better equipped than we
are to make the pivotal policy choice between a mechanism-
focused and shooter-focused understanding of “function of the
trigger.” And the Bureau’s interpretation comports with how
some courts have read the statute, which is a strong sign of
reasonableness. In United States v. Akins, 312 F. App’x 197
(11th Cir. 2009), for example, the Eleventh Circuit held that the
Bureau’s reading of “single function of the trigger” to mean
“single pull of the trigger” was “consonant with the statute and
its legislative history.” Id. at 200. The court relied on that
definition to conclude that an “Accelerator”—a type of bump
stock—was reasonably classified as a machine gun. Id. And
“single pull of the trigger” has been the definition the agency
has employed since 2006. See 83 Fed. Reg. at 66,543.
                               52
     The Rule’s interpretation also accords with how the phrase
“single pull of the trigger” was understood at the time of the
enactment of the National Firearms Act. See 83 Fed. Reg. at
66,518. The Rule cites a congressional hearing for the National
Firearms Act in which the then-president of the National Rifle
Association testified that the term “machine gun” included any
gun “capable of firing more than one shot by a single pull of
the trigger, a single function of the trigger.” 83 Fed. Reg.
66,518. And the House Report accompanying the bill that
eventually became the National Firearms Act states that the bill
“contains the usual definition of a machine gun as a weapon
designed to shoot more than one shot * * * by a single pull of
the trigger.” H.R. Rep. No. 73-1780, at 2 (1934).

     The Bureau’s interpretation of “automatically” is
permissible too. The Rule’s requirement of a “self-acting or
self-regulating mechanism” demands a significant degree of
autonomy from the weapon without mandating a firing
mechanism that is completely autonomous. That definition
accords with the everyday understanding of the word
“automatic.” And it focuses the inquiry about what needs to be
automated right where the statute does: the ability of the trigger
function to produce “more than one shot, without manual
reloading.”     26 U.S.C. § 5845(b).       It also tracks the
interpretation reached by the Seventh Circuit in United States
v. Olofson, 563 F.3d 652 (7th Cir. 2009), in which the court
interpreted the term to require a “self-acting mechanism”
without requiring more, id. at 658.

    The plaintiffs argue that the Bureau’s definition of
“machinegun” is unreasonable because it has the effect of
reaching all semiautomatic rifles. Because “virtually all”
semiautomatic rifles can be “bump-fired” with the use of
common household items, the plaintiffs contend, the Bureau’s
                               53
definition covers even unmodified semiautomatic rifles, which
renders it unreasonable. Guedes Br. 18.

     The Rule explains why the plaintiff’s understanding is
incorrect, and the Rule’s explanation in that regard is
reasonable. See 83 Fed. Reg. at 66,532–66,534. The Bureau
acknowledges that bump firing—a technique using a stable
point like a belt loop to approximate the function of a bump
stock—is possible with semiautomatic weapons. See id. at
66,533. But even when a semiautomatic weapon is bump fired
using an object like a belt loop or a rubber band, the Bureau
explained, the weapon does not fire “automatically” because
there is no “self-acting or self-regulating mechanism.” Rubber
bands and their ilk do not “capture and direct the recoil energy”
to “harness[] [it] as part of a continuous back-and-forth cycle.”
Id. at 66,533. Rather, “the shooter must do so” herself. Id.
Bump firing without the aid of a bump-stock-type device is
therefore “more difficult” because it relies solely on the shooter
“to control the distance that the firearm recoils and the
movement along the plane on which the firearm recoils.” Id.

     Bump stocks, on the other hand, are specifically designed
to “direct[] the recoil energy of the discharged rounds * * * in
constrained linear rearward and forward paths.” Id. at 66,532.
By capturing the recoil energy of the gun and directing it
through a specified “distance” and along a specified “plane,”
bump stocks “incorporate[] a self-acting or self-regulating
component” that would otherwise be absent. Id. at 66,533.
Thus, belt loops, unlike bump stocks, do not transform
semiautomatic weapons into statutory “machineguns.” Or so
the Bureau reasonably concluded in the Rule.

    “If a statute is ambiguous, and if the implementing
agency’s construction is reasonable, Chevron requires a federal
court to accept the agency's construction of the statute, even if
                               54
the agency's reading differs from what the court believes is the
best statutory interpretation.” Brand X, 545 U.S. at 980. Here,
the Bump-Stock Rule sets forth a permissible interpretation of
the statute’s ambiguous definition of “machinegun.” It
therefore merits our deference.

                                C

     In addition to their argument that the Rule is incompatible
with the statutory definition of a machine gun, the plaintiffs
also contend that the Rule is arbitrary and capricious. Agency
action is arbitrary or capricious if “the agency has relied on
factors which Congress has not intended it to consider, entirely
failed to consider an important aspect of the problem, offered
an explanation for its decision that runs counter to the evidence
before the agency, or is so implausible that it could not be
ascribed to a difference in view or the product of agency
expertise.” Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto.
Ins. Co., 463 U.S. 29, 43 (1983). Here, the plaintiffs claim that,
for various reasons, the Rule is arbitrary in applying the
statutory definition of “machinegun” to bump stocks. None of
their claims is likely to succeed.

     First, the plaintiffs argue that the Rule fundamentally
mischaracterizes the operation of bump-stock devices. In their
view, the Rule disregards that, for each shot, “the shooter must
manually and volitionally push the trigger into [a] stationary
finger.” Guedes Br. 24. It is true that, for a bump-stock-
equipped device to repeatedly fire, the shooter must keep the
bumpstock engaged by maintaining constant forward pressure
on the gun. But in the Rule, the Bureau correctly describes the
operation of bump-stock-equipped devices: the shooter must
“maintain[] constant forward pressure [on the gun] with the
non-trigger hand” in order to maintain continuous fire. 83 Fed.
Reg. at 66,532. The bump stock takes advantage of the gun’s
                               55
recoil, channeled into a linear back-and-forth cycle, to permit
the shooter to fire continuously by maintaining steady forward
pressure on the gun. There is thus no disagreement about the
basic mechanics of bump-stock devices.

     Guedes takes particular issue with the Rule’s
characterization of recoil. He argues that bump-stock-
equipped devices cannot “harness[] the recoil energy of the
firearm” because they do not use “a device such as a spring or
hydraulics * * * [to] automatically absorb the recoil and use
this energy to activate itself.” Guedes Br. 16–17. But the Rule
does not adopt such an impoverished definition of
“automatically.” The Rule requires only that the recoil be used
in service of a “self-acting or self-regulating mechanism.” A
bump stock “direct[s] the recoil energy of the discharged
rounds * * * in constrained linear rearward and forward paths,”
83 Fed. Reg. at 66,518 (quoting 83 Fed. Reg. at 13,443), which
qualifies as a “self-regulating mechanism.”

     Second, the plaintiffs assert that the Rule is arbitrary
because its definition encompasses all semiautomatic weapons.
That argument is largely redundant of the plaintiffs’ Chevron
step two argument to the same effect, which we have already
addressed. We dispose of this iteration of the same argument
on the same grounds: Bump stocks, unlike commonplace
household objects, are specifically designed to “direct[] the
recoil energy of the discharged rounds * * * in constrained
linear rearward and forward paths.” Id. Bump stocks, unlike
household objects, are machine guns because they alone
involve a “self-acting or self-regulating mechanism.” Id.

     Third, the plaintiffs submit that the Rule arbitrarily
excludes binary-trigger guns from its definition of
“machinegun.” Binary-trigger guns shoot one round when the
trigger is pulled and another round when the trigger is released.
                                56
83 Fed. Reg. at 66,534. The Rule concludes that such devices
are not machine guns because the second shot is “the result of
a separate function of the trigger.” Id. The plaintiffs argue that
if the release of the trigger is a separate function, the operation
of a bump stock—which requires the shooter to keep the trigger
finger stationary while steadily pushing the gun forward into
the finger—must also involve multiple functions of the trigger.
But the Rule reasonably distinguishes binary-trigger guns on
the ground that they require a second act of volition with the
trigger finger. The release of a trigger is a volitional motion.
But merely holding the trigger finger stationary—which is
what operation of a bump stock entails—is not.

     Fourth, Guedes contends that the Rule is arbitrary because
its definition of “automatically” is ambiguous. The Rule’s
definition, Guedes notes, does not specify how much manual
input is too much. But the existence of latent ambiguity does
not render an interpretation arbitrary or capricious. Agencies
are permitted to promulgate regulations interpreting
ambiguous statutes without having to resolve all possible
ambiguity.

     Fifth, Codrea argues that the Rule arbitrarily failed to
consider reliance interests, “an important aspect of the
problem.” State Farm, 463 U.S. at 43. It is true that “the APA
requires an agency to provide more substantial justification
when * * * its prior policy has engendered serious reliance
interests that must be taken into account.” Perez, 135 S.Ct. at
1209 (quoting FCC v. Fox Television Stations, Inc., 556 U.S.
502, 515 (2009)). But the only reliance interest identified by
Codrea is the pecuniary interest of current possessors of bump-
stock devices. See Codrea Br. 19–20 & 19 n.4; Comment of
Maryland Shall Issue at 6. And in the Rule, the Bureau
engaged in a cost-benefit analysis that considered, among other
                               57
things, the cost incurred by owners of bump-stock devices. See
83 Fed. Reg. at 66,546.

     Finally, Guedes argues that the Rule is arbitrary because it
is the product of “naked political desire.” Guedes Br. 18.
Insofar as Guedes means to claim that the Rule arises from
political considerations, he is surely right. All would agree that
the Bureau enacted this Rule in response to the urging of “the
President, Members of Congress, and others,” as part of an
“immediate and widespread” outcry in the wake of the 2017
mass shooting in Las Vegas. Guedes, 356 F. Supp. 3d at 120,
123. The Rule itself describes its origins in a memorandum
issued by President Trump to then–Attorney General Sessions
“direct[ing] the Department of Justice * * * ‘as expeditiously
as possible, to propose for notice and comment a rule banning
all devices that turn legal weapons into machineguns.’” 83
Fed. Reg. at 66,516–66,517 (quoting Application of the
Definition of Machinegun to ‘Bump Fire’ Stocks and Other
Similar Devices, 83 Fed. Reg. 7,949 (Feb. 23, 2018)). But that
is hardly a reason to conclude that the Rule is arbitrary.
Presidential administrations are elected to make policy. And
“[a]s long as the agency remains within the bounds established
by Congress, it is entitled to assess administrative records and
evaluate priorities in light of the philosophy of the
administration.” Nat’l Ass’n of Home Builders v. EPA, 682
F.3d 1032, 1043 (D.C. Cir. 2012) (quoting State Farm, 463
U.S. at 59 (Rehnquist, J., concurring in part and dissenting in
part)).

     Guedes might instead mean to contend that the Bureau was
so eager to enact the policy preferences of the President that it
failed to engage in reasoned consideration of the issues. The
central purpose of arbitrary or capricious review is to assure
that the agency has engaged in “reasoned decisionmaking.”
State Farm, 463 U.S. at 52. We ordinarily do so, however, by
                                 58
examining whether the agency has “articulate[d] a satisfactory
explanation for its actions.” Id. at 43. Here, the agency has
articulated a satisfactory explanation for the Bump-Stock Rule.
And the administrative record reflects that the agency kept an
open mind throughout the notice-and-comment process and
final formulation of the Rule. See Air Transp. Ass’n of Am. v.
Nat’l Mediation Bd., 663 F.3d 476, 487–488 (D.C. Cir. 2011);
C&W Fish Co. v. Fox, 931 F.2d 1556, 1564–1565 (D.C. Cir.
1991). In the absence of any actual evidence of delinquent
conduct, we accord the Bureau a “presumption of regularity”
in its promulgation of the Rule. Citizens to Pres. Overton Park,
Inc. v. Volpe, 401 U.S. 402, 415 (1971).

                                 D

     Finally, Codrea argues that the Rule must be vacated
because it is impermissibly retroactive, violating both 26
U.S.C. § 7805(b)’s bar on retroactive rulemaking and the Ex
Post Facto Clause. That claim has been forfeited because the
plaintiffs failed to raise it in the district court. The Rule, at any
rate, cannot be characterized as retroactive: As we have
explained, the Rule itself made clear that the possession of
bump stocks would become unlawful only after the effective
date.

     Further, it matters not that the government’s post hoc
litigation strategy has been to characterize the Rule as merely
interpretive and, consequently, backward looking.
Irrespective of that litigating position, the Rule is legislative in
character and therefore purely prospective. Any criminal
consequences did not attach until the Rule’s effective date.
And notice to the public has been clear and explicit.
                              59
                         * * * * *

     The plaintiffs have failed to establish a likelihood of
success both for their challenge to Acting Attorney General
Whitaker’s appointment and for their objections to the
substantive validity of the Rule. For the foregoing reasons, we
affirm the district court’s denial of a preliminary injunction.

                                                   So ordered.
     KAREN LECRAFT HENDERSON, Circuit Judge, concurring
in part and dissenting in part: Federal law makes it a crime to
possess or transfer a “machinegun.” 18 U.S.C. § 922(o)(1).
This case is about a Bureau of Alcohol, Tobacco, Firearms, and
Explosives (ATF) regulation that reinterprets the statutory
definition of “machinegun” and applies it to all bump stock
type devices. Bump-Stock-Type Devices, 83 Fed. Reg. 66,514
(Dec. 26, 2018) (Bump Stock Rule or Rule). Individual
firearms owners and non-profit groups sued the ATF, seeking
preliminary injunctive relief to stop the Bump Stock Rule from
going into effect. The issue before us on this expedited appeal
of the district court’s denial of preliminary injunctive relief,
Guedes v. ATF, 356 F. Supp. 3d 109 (D.D.C. 2019), is whether
the plaintiffs are likely to succeed on the merits of their
challenge to the Bump Stock Rule as contrary to the statutory
definition of “machinegun.” Unlike my colleagues, I believe
the Bump Stock Rule does contradict the statutory definition
and, respectfully, part company with them on this issue. 1

     A “machinegun” is a firearm “which shoots . . .
automatically more than one shot, without manual reloading,
by a single function of the trigger.” 26 U.S.C. § 5845(b). In
my view, the Rule impermissibly adds to the language
“automatically . . . by a single function of the trigger,”
including within its definition a firearm that shoots more
rapidly only by a single function of the trigger and the shooter’s
additional manual input. The statute specifies a single
function; the Rule specifies a single function plus. “Whether
the Government interprets a criminal statute too broadly (as it
sometimes does) or too narrowly,” we have “an obligation to
correct its error.” Abramski v. United States, 573 U.S. 169, 191
(2014).




    1
        I concur in Parts II and III.A of the majority opinion.
                                 2
                        I. BACKGROUND

                   A. Statutory Framework

     The National Firearms Act of 1934, Pub. L. No. 73-474,
48 Stat. 1236, “imposes strict registration requirements on
statutorily defined ‘firearms.’” Staples v. United States, 511
U.S. 600, 602 (1994). In the 1934 legislation, the Congress
defined “machinegun” as a specific type of “firearm.” The
original text defined a “machinegun” as “any weapon which
shoots, or is designed to shoot, automatically or
semiautomatically, more than one shot, without manual
reloading, by a single function of the trigger.” National
Firearms Act § 1(b). A few decades later, the Gun Control Act
of 1968, Pub. L. No. 90-618, 82 Stat. 1213, amended the
definition in two key ways, deleting the phrase “or
semiautomatically” and including “parts” designed and used to
“convert a weapon into a machinegun.” 2 Gun Control Act, tit.
II, § 201, 82 Stat. at 1231 (codified at 26 U.S.C. § 5845(b)).
The definition of “machinegun” in effect today includes “any
weapon which shoots, is designed to shoot, or can be readily
restored to shoot, automatically more than one shot, without
manual reloading, by a single function of the trigger.” 26
U.S.C. § 5845(b).

    The Firearms Owners’ Protection Act of 1986 (Act), Pub.
L. No. 99-308, 100 Stat. 449, effectively banned private
ownership of machine guns. Firearms Owners’ Protection Act,
§ 102(9) (codified at 18 U.S.C. § 922(o)(1)). The Act makes it

    2
       It thus extends to “the frame or receiver of any such weapon,
any part designed and intended solely and exclusively, or
combination of parts designed and intended, for use in converting a
weapon into a machinegun, and any combination of parts from which
a machinegun can be assembled if such parts are in the possession or
under the control of a person.” 26 U.S.C. § 5845(b).
                               3
“unlawful for any person to transfer or possess a machinegun,”
18 U.S.C. § 922(o)(1)), and “machinegun” has “the meaning
given . . . in section 5845(b) of the National Firearms Act,” id.
§ 921(a)(3). A person who “knowingly” violates the ban can
be “fined . . . [or] imprisoned not more than 10 years, or both.”
Id. § 924(a)(2). The ban has two exceptions: one for “a transfer
to or by, or possession by or under the authority of” the federal
government or a state government, id. § 922(o)(2)(A), and the
other grandfathers any “machinegun” lawfully possessed
before the Act went into effect, id. § 922(o)(2)(B).

           B. History of Bump Stock Regulation

     Firearms manufacturers have created various devices that
allow a lawful semiautomatic rifle to perform more rapidly. A
bump stock is one such device. It replaces the standard stock
of a rifle—the part that rests against the shooter’s shoulder. A
bump stock “free[s] the weapon to slide back and forth
rapidly.” Bump-Stock-Type Devices, 83 Fed. Reg. at 66,516.
The sliding motion allows a shooter to increase his rate of fire.
A rifle produces recoil energy upon firing. The bump stock
helps direct the firearm’s recoil and convert the recoil energy
into rapidly firing rounds. It works like this: the shooter pulls
the trigger; the recoil causes the firearm to slide backward; the
shooter maintains backward pressure on the trigger with the
index finger of his shooting hand and forward pressure on the
barrel with his other hand. Id. This process causes the firearm
to slide back and forth rapidly, bumping the shooter’s
stationary trigger finger and thereby firing additional rounds.
Id.

     Some bump stock devices use only the shooter’s physical
pressure to channel the recoil energy and do not include springs
or mechanical parts. Id. For these devices, a single pull of the
trigger alone—without the shooter’s additional forward
                                 4
pressure—does not cause the firearm to shoot more than one
round. Video evidence in the record makes this clear. 3 In the
video, the shooter fires a rifle equipped with a non-mechanical
bump stock. The shooter holds the rifle with one hand, the
trigger hand. He then pulls the trigger and the rifle fires a single
shot. Without his other hand’s forward pressure on the barrel,
the rifle equipped with a non-mechanical bump stock fires only
a single round with each pull of the trigger.

     The ATF first classified a bump stock type device in 2002,
concluding that it was not a “machinegun.” Id. at 66,517. The
classification involved a product called the Akins Accelerator,
a bump stock that used internal springs. “To operate the device,
the shooter initiated an automatic firing sequence by pulling the
trigger one time, which in turn caused the rifle to recoil within
the stock, permitting the trigger to lose contact with the finger
and manually reset.” Id. “Springs in the Akins Accelerator
then forced the rifle forward, forcing the trigger against the
finger, which caused the weapon to discharge the ammunition.”
Id. The ATF interpreted the statutory language “single
function of the trigger” to mean a “single movement of the
trigger.” Id. A semi-automatic rifle fires only a single round
each time the trigger is pulled and reset. According to the ATF,
because the Akins Accelerator did not modify how a
semiautomatic rifle’s trigger “moves” with each shot, it was not
a “machinegun.”

     In 2006, the ATF reclassified the Akins Accelerator as a
“machinegun.” It reinterpreted the phrase “single function of
the trigger” from “single movement of the trigger” to “single
pull of the trigger.” Id. The reinterpretation made all the
difference. Once a shooter pulls and maintains pressure on the
trigger, the internal springs of the Akins Accelerator start an
    3
      The declaration of Rick Vasquez, a former senior ATF
Technical Expert, attests to the accuracy of the video evidence.
                               5
automatic sequence that keeps the rifle firing until the shooter
removes his finger or depletes the ammunition. The firing of
multiple rounds based on a single continuous pull of the trigger
made the device a “machinegun” under the ATF’s
reinterpretation. The Akins Accelerator inventor challenged
the ATF’s changed reading in federal district court (M.D. Fla.),
arguing that the Agency misinterpreted the statutory definition
of “machinegun.” The district court upheld the ATF’s
determination and the Eleventh Circuit affirmed. Akins v.
United States, 312 F. App’x 197 (11th Cir. 2009). The
appellate court concluded that “the interpretation by the Bureau
that the phrase ‘single function of the trigger’ means a ‘single
pull of the trigger’ is consonant with the [National Firearms
Act] and its legislative history.” Id. at 200 (quoting 26 U.S.C.
§ 5845(b)).

     “In ten letter rulings between 2008 and 2017, ATF applied
the ‘single pull of the trigger’ interpretation to other bump-
stock-type devices” and determined that none qualified as a
“machinegun.” Bump-Stock-Type Devices, 83 Fed. Reg. at
66,517. Although each device fired more than one round with
a single pull of the trigger, the ATF concluded that none was a
“machinegun” because the firing sequence did not occur
“automatically.” Unlike the Akins Accelerator, the devices did
not rely on springs or mechanical parts. In order to use them,
“the shooter [had to] apply constant forward pressure with the
non-shooting hand and constant rearward pressure with the
shooting hand.” Joint Appendix (J.A.) at 278. Thus, the ATF
drew a distinction between a bump stock with mechanical parts
like springs that cause a more rapid firing sequence and a bump
stock that uses both of the shooter’s hands to do the same. E.g.,
Letter from Richard W. Marianos, Assistant Dir. Pub. and
Governmental Affairs, to Congressman Ed Perlmutter (April
16, 2013), reprinted at J.A. 281–82.
                                 6
                   C. The Bump Stock Rule

     In October 2017, a gunman armed with several
semiautomatic rifles killed 58 people and wounded 500 more
in Las Vegas, Nevada. The rifles were equipped with bump
stock devices, which “were readily available in the commercial
marketplace through online sales directly from the
manufacturer, and through multiple retailers.” Bump-Stock-
Type Devices, 83 Fed. Reg. at 66,514. Using these devices, the
gunman was able to fire hundreds of rounds in a matter of
minutes. Within months, the ATF began to promulgate a
regulation to classify any bump stock type device as a
“machinegun.” President Trump directed the DOJ to “dedicate
all available resources to . . . propos[ing] for notice and
comment a rule banning all devices that turn legal weapons into
machineguns.” Application of the Definition of Machinegun to
“Bump Fire” Stocks and Other Similar Devices, 83 Fed. Reg.
7,949 (Feb. 20, 2018).

     In December 2018, the ATF promulgated the Bump Stock
Rule. 4 Bump-Stock-Type Devices, 83 Fed. Reg. at 66,514. It
declares that all bump stock type devices “are ‘machineguns’
as defined by the National Firearms Act of 1934 and the Gun
Control Act of 1968 because such devices allow a shooter of a
semiautomatic firearm to initiate a continuous firing cycle with
a single pull of the trigger.” Id. According to the Rule, the
“devices convert an otherwise semiautomatic firearm into a
machinegun by functioning as a self-acting or self-regulating
mechanism that harnesses the recoil energy of the
semiautomatic firearm in a manner that allows the trigger to
reset and continue firing without additional physical
manipulation of the trigger by the shooter.” Id. (emphasis

    4
       The Rule amends three separate regulations, 27 C.F.R.
§§ 447.11, 478.11, 479.11, reinterpreting with identical language the
statutory definition of “machinegun” in each.
                                 7
added). Thus, “a semiautomatic firearm to which a bump-stock
device is attached is able to produce automatic fire with a single
pull of the trigger.” Id.

     The Bump Stock Rule was scheduled to go into effect on
March 26, 2019. 5 There were then an estimated 280,000 to
520,000 previously legal bump stocks in circulation in the
United States. See Bump-Stock-Type Devices, 83 Fed. Reg.
13,442, 13,451 (March 29, 2018). Under the Rule, “[b]ump-
stock-type devices . . . possessed by individuals [had] to be
destroyed or abandoned” before March 26. Bump-Stock-Type
Devices 83 Fed. Reg. at 66,546. Anyone who possesses or
transports the device after that date faces criminal liability. 18
U.S.C. § 922(o)(1).

                     D. Procedural History

     The plaintiffs, five individual firearms owners and four
non-profit organizations, challenge the Bump Stock Rule’s
legality on several grounds. Their primary challenge is that the
Rule misinterprets the statutory definition of “machinegun”
and mistakenly extends that definition to cover bump stock
type devices. They also attack the Rule for alleged procedural
gaps in the rulemaking process and for taking property without
just compensation in violation of the Fifth Amendment’s Due
Process Clause. Finally, the plaintiffs contend that former
Acting Attorney General Matthew Whitaker was not properly
appointed to his position and thus lacked authority to approve
the Rule. The plaintiffs separately moved for preliminary
injunctive relief.


    5
       After hearing argument on March 22, 2019, we issued an
administrative order staying the Rule’s effective date but only as to
the plaintiffs. Per Curiam Order, Guedes v. ATF, No. 19-5042 (D.C.
Cir. March 23, 2019).
                                8
     The district court consolidated and denied the motions. “A
plaintiff seeking a preliminary injunction must establish that he
is likely to succeed on the merits, that he is likely to suffer
irreparable harm in the absence of preliminary relief, that the
balance of equities tips in his favor, and that an injunction is in
the public interest.” Winter v. Nat. Res. Def. Council, Inc., 555
U.S. 7, 20 (2008). The district court determined that the Rule
reasonably interprets “machinegun” to include bump stock
devices. It also rejected the plaintiffs’ other challenges either
as unlikely to succeed on the merits or as unsuitable for
equitable relief. Accordingly, the district court denied relief
without reaching the other three preliminary-injunction factors.
The plaintiffs then filed a timely interlocutory appeal. See 28
U.S.C. § 1292(a)(1).

                         II. ANALYSIS

                  A. STANDARD OF REVIEW

     The district court’s denial of preliminary injunctive relief
rests on its legal determination that the Bump Stock Rule does
not misinterpret or misapply the statutory definition of
“machinegun.” Our review is therefore de novo. City of Las
Vegas v. Lujan, 891 F.2d 927, 931–32 (D.C. Cir. 1989) (de
novo review of denial of preliminary injunctive relief where
“district judge did not make any factual determinations . . .
since he was sitting in appellate review of agency action” and
“denied the preliminary injunction because, and only because,
he believed the [agency] was likely to succeed on the merits”);
see also Athens Cmty. Hosp., Inc. v. Shalala, 21 F.3d 1176,
1178 (D.C. Cir. 1994) (“Upon the issue whether an
administrative regulation is lawful, we do not defer to the
judgment of the district court.”).

     Despite the parties’ agreement that the de novo standard of
review applies, my colleagues, like the district court, see
                                  9
Guedes, 356 F. Supp. 3d at 126–27, nonetheless review the
ATF’s interpretation under the two-step framework set out in
Chevron, U.S.A., Inc. v. Natural Resources Defense Council,
Inc., 467 U.S. 837, 842–43 (1984). 6 But the United States
Supreme Court has recently clarified whether the Chevron
framework applies to a statute—and, by extension a rule—
enforced by a criminal sanction. United States v. Apel, 571
U.S. 359, 369 (2014) (“[W]e have never held that the
Government’s reading of a criminal statute is entitled to any
deference.”). In another recent decision, Abramski v. United
States, the ATF had taken one view of 18 U.S.C. § 922(a)(6)
for “almost two decades,” concluding that a straw purchaser’s
“misrepresentation” counted as “material” under the statute
notwithstanding the true buyer could legally possess a gun.
573 U.S. at 191. The defendant pointed out that the ATF had
until 1995 taken the opposite position, requiring the true buyer
to be ineligible to possess a gun in order to make the straw
purchaser’s misrepresentation “material.” Id. The Supreme
Court responded that the “ATF’s old position [is] no more
     6
        Even under Chevron, “[a]n agency construction of a statute
cannot survive judicial review if a contested regulation reflects an
action that exceeds the agency’s authority.” Aid Ass’n for Lutherans
v. U.S. Postal Serv., 321 F.3d 1166, 1174 (D.C. Cir. 2003). Because
the Bump Stock Rule exceeds the ATF’s authority by veering from
the plain meaning of the statute, I would reach the same conclusion
whether Chevron step one or de novo review applies.

     In reply to my colleagues’ insistence that, at the rulemaking
stage, the ATF emphasized its reliance on Chevron, Maj. Op. at 26–
28, I would note that the ATF in fact declared that the Rule’s
interpretations of “single function of the trigger” and “automatically”
“accord with the plain meaning of those terms.” Bump-Stock-Type
Devices, 83 Fed. Reg. at 66,527 (emphasis added). Its “fallback”
position at that stage was “even if those terms are ambiguous, this
rule rests on a reasonable construction of them.” Id. (emphasis
added).
                                  10
relevant than its current one—which is to say, not relevant at
all.” Id. Indeed, “[w]hether the Government interprets a
criminal statute too broadly (as it sometimes does) or too
narrowly (as the ATF used to in construing § 922(a)(6)), a court
has an obligation to correct its error.” Id. In its Apel and
Abramski decisions, then, “[t]he Supreme Court has expressly
instructed us not to apply Chevron deference when an agency
seeks to interpret a criminal statute.” Gutierrez-Brizuela v.
Lynch, 834 F.3d 1142, 1156 (10th Cir. 2016) (Gorsuch, J.,
concurring).

      My colleagues believe that this case is different because
the 26 U.S.C. § 5845(b) definition of “machinegun” has both
civil 7 and criminal 8 enforcement implications. They reach
their conclusion regarding the applicable standard of review
based in part on a footnote in Babbitt v. Sweet Home Chapter
of Communities for a Great Oregon, 515 U.S. 687 (1995). That
case involved a regulation interpreting the definition of “harm”
under the Endangered Species Act, a regulation with both
criminal and civil enforcement implications. Id. at 704 n.18.


     7
        See 26 U.S.C. § 5872(a) (“Any firearm involved in any
violation of the provisions of this chapter shall be subject to seizure
and forfeiture, and (except as provided in subsection (b)) all the
provisions of internal revenue laws relating to searches, seizures, and
forfeitures of unstamped articles are extended to and made to apply
to the articles taxed under this chapter, and the persons to whom this
chapter applies.”)
     8
      See 18 U.S.C. § 922(o)(1) (“[I]t shall be unlawful for any
person to transfer or possess a machinegun.”); 18 U.S.C.
§ 921(a)(23) (“The term ‘machinegun’ has the meaning given such
term in section 5845(b) of the National Firearms Act (26 U.S.C.
5845(b)).”); 18 U.S.C. § 924(a) (establishing penalties for
“knowing[]” or “willful[]” violation of, inter alia, section
922(o)(1)’s ban on machinegun possession or transfer).
                                 11
The Supreme Court deferred to the Secretary of the Interior’s
interpretation under Chevron. Id. at 703–04. The majority
reads Babbitt—and some of our precedent—to establish a
bright-line rule that any regulation with both civil and criminal
enforcement provisions merits Chevron deference. Maj. Op. at
36–40; see In re Sealed Case, 223 F.3d 775, 779 (D.C. Cir.
2000); United States v. Kanchanalak, 192 F.3d 1037, 1047 n.17
(D.C. Cir. 1999). 9

     With respect, I am not convinced that my colleagues’
reading of Babbitt as the last word on this topic is correct. See
Solid Waste Agency of N. Cook Cty. v. U.S. Army Corps of
Eng’rs, 531 U.S. 159, 174 n.8 (2001) (declining, post-Babbitt,
to address relationship between Chevron and agency regulation
interpreting statute with criminal sanction). The Supreme
Court’s most recent decisions indicate, as the ATF and the
plaintiffs argue here, Government Br. 36–37; Codrea Opening
Br. at 9–11, that Chevron review does not apply to a statute/rule
with criminal sanctions. 10 Apel, 571 U.S. at 369; Abramski,
573 U.S. at 191. And if Chevron review does not apply to a
    9
        One post-Apel and Abramski Circuit decision applies the
Chevron framework to a regulation with criminal and civil
enforcement provisions. Competitive Enter. Inst. v. U.S. Dep’t of
Transp., 863 F.3d 911, 915 (D.C. Cir. 2017). But only one judge
signed on to that view; one dissented and another wrote separately to
explain that he would reach the same result under de novo review,
which made Chevron’s applicability vel non unnecessary to his vote,
id. at 921 (Kavanaugh, J., concurring).
    10
       I leave for another day whether the Government can “waive”
Chevron review, as my colleagues view the ATF’s stance here. Maj.
Op. at 32–36; but see Glob. Tel*Link v. FCC, 866 F.3d 397, 417
(D.C. Cir. 2017) (“it would make no sense for this court to determine
whether” agency action “warrant[s] Chevron deference” if the
agency “no longer seeks deference”). I view the ATF’s stance to be
that Chevron is inapplicable—period. Government Br. 36–37.
                               12
statute/rule with criminal sanctions, Chevron cannot apply to a
statute/rule with both criminal and civil sanctions. See Clark
v. Martinez, 543 U.S. 371, 380 (2005) (a statute can have only
a single meaning and “[t]he lowest common denominator, as it
were, must govern”); Leocal v. Ashcroft, 543 U.S. 1, 11 n.8
(2004) (“[W]e must interpret [a] statute consistently, whether
we encounter its application in a criminal or noncriminal
context.”). Again, with respect, the majority may misread
Babbitt, which itself includes language that can allow its
holding to be reconciled with recent Supreme Court decisions:

       We have never suggested that the rule of lenity
       should provide the standard for reviewing facial
       challenges to administrative regulations
       whenever the governing statute authorizes
       criminal enforcement. Even if there exist
       regulations whose interpretations of statutory
       criminal penalties provide such inadequate
       notice of potential liability as to offend the rule
       of lenity, the “harm” regulation, which has
       existed for two decades and gives a fair warning
       of its consequences, cannot be one of them.

515 U.S. at 704 n.18. Footnote 18 suggests, I submit, that a
regulation with a criminal sanction can violate the rule of lenity
but concluded that the regulation at issue, with its longstanding
definition of “harm,” did not do so. Id. My reading allows
Babbitt to be harmonized with more recent decisions: Chevron
does not apply to a regulation enforced both civilly and
criminally unless the regulation gives fair warning sufficient to
avoid posing a rule of lenity problem.                The ATF’s
interpretation of “machinegun” gives anything but fair
warning—instead, it does a volte-face of its almost eleven
years’ treatment of a non-mechanical bump stock as not
constituting a “machinegun.”
                                13
      Although I do not dispute that the ATF has been delegated
general rulemaking authority to implement section 5845(b),
inter alia, I am less certain than my colleagues that we owe
deference to the ATF’s interpretation of section 5845(b).
“Deference under Chevron to an agency’s construction of a
statute that it administers is premised on the theory that a
statute’s ambiguity constitutes an implicit delegation from
Congress to the agency to fill in the statutory gaps.” FDA v.
Brown & Williamson Tobacco Corp., 529 U.S. 120, 159
(2000). Statutory ambiguity, if it exists, does not necessarily
constitute an implicit delegation. King v. Burwell, 135 S. Ct.
2480, 2488–89 (2015); United States Telecom Ass’n v. FCC,
855 F.3d 381, 419–24 (D.C. Cir. 2017) (Kavanaugh, J.,
dissenting from denial of rehearing en banc). The Congress
must, for instance, “speak clearly if it wishes to assign to an
agency decisions of vast economic and political significance.”
Util. Air Regulatory Grp. v. EPA, 573 U.S. 302, 324 (2014)
(quotation marks omitted). There is good reason to believe that
a similar clear-statement rule applies in the criminal law
context. Under longstanding separation-of-powers principles,
the Congress defines the criminal law and must speak distinctly
to delegate its responsibility. 11 United States v. Bass, 404 U.S.
336, 348 (1971); United States v. Grimaud, 220 U.S. 506, 519,
522 (1911); United States v. Eaton, 144 U.S. 677, 688 (1892).
Unlike with civil statutes, then, ambiguity in the criminal law
is presumptively for the Congress—not the ATF—to resolve.
Whitman v. United States, 135 S. Ct. 352, 354 (2014) (Scalia,
J., statement respecting denial of certiorari) (“Congress cannot,
through ambiguity, effectively leave that function to the
courts—much less to the administrative bureaucracy.”).
    11
        The Supreme Court has upheld executive branch
interpretations of the criminal law based on express delegations of
interpretive authority. See United States v. O’Hagan, 521 U.S. 642,
667 (1997) (Securities Exchange Act of 1934); Touby v. United
States, 500 U.S. 160, 165–69 (1991) (Controlled Substances Act).
                               14
Accordingly, I would treat an ambiguous criminal statute to be
of “vast economic and political significance” and apply
Chevron only if the Congress expressly delegates its
lawmaking responsibility. See Util. Air Regulatory Grp., 573
U.S. at 324. The Congress has made no such clear statement;
instead the ATF relies solely on its general rulemaking power
and statutory ambiguity. 18 U.S.C. § 926(a); 26 U.S.C. §§
7801(a)(2)(A), 7805(a). Chevron is inapplicable. See King,
135 S. Ct. at 2489.

     I believe the applicable standard of review is de novo and
therefore we should go “the old-fashioned” route and “decide
for ourselves the best reading” of “machinegun.” Miller v.
Clinton, 687 F.3d 1332, 1342 (D.C. Cir. 2012) (quoting
Landmark Legal Found. v. IRS, 267 F.3d 1132, 1136 (D.C. Cir.
2001)). As is always the case in construing a statute, the
inquiry focuses on “the plain meaning of the text, looking to
the ‘language itself, the specific context in which that language
is used, and the broader context of the statute as a whole.’”
Blackman v. District of Columbia, 456 F.3d 167, 176 (D.C. Cir.
2006) (quoting United States v. Barnes, 295 F.3d 1354, 1359
(D.C. Cir. 2002)). The Bump Stock Rule declares that any
bump stock device qualifies as a “machinegun.” Although the
Rule—in my view—correctly interprets “single function of the
trigger,” it misreads “automatically.” Moreover, it misapplies
its interpretation of “single function of the trigger” to bump
stock type devices.

            B. “Single Function of the Trigger”

     The Rule determines that “single function of the trigger”
within the statutory definition of “machinegun” means “single
pull of the trigger and analogous motions.” Bump-Stock-Type
Devices, 83 Fed. Reg. at 66,554. To me, the “function” of the
trigger means “action” of the trigger. Webster’s New
                               15
International Dictionary 1019 (2d ed. 1934). According to the
section 5845(b) definition, the trigger function “shoots” the
firearm. 26 U.S.C. § 5845(b) (“The term ‘machinegun’ means
any weapon which shoots . . . automatically more than one
shot, without manual reloading, by a single function of the
trigger.”); see also Fortier v. Olin Corp., 840 F.2d 98, 101 (1st
Cir. 1988) (discussing mechanics of lever-action rifle). “Pull
of the trigger,” then, describes how the trigger works. See
Staples, 511 U.S. at 602 n.1; United States v. Camp, 343 F.3d
743, 745 (5th Cir. 2003) (using trigger “pull” and “function”
interchangeably); United States v. Oakes, 564 F.2d 384, 388
(10th Cir. 1977) (same). The Rule recognizes that not all
firearms feature a pull trigger; some involve “fire initiated by
voice command, electronic switch, swipe on a touchscreen or
pad, or any conceivable number of interfaces.” Bump-Stock-
Type Devices, 83 Fed. Reg. at 66,534; see also United States v.
Fleischli, 305 F.3d 643, 655–56 (7th Cir. 2002) (minigun fired
by “electronic switch” is machinegun). To include these non-
pull methods used to shoot a firearm, the Rule includes the
phrase “and analogous motions.” Bump-Stock-Type Devices,
83 Fed. Reg. at 66,553.

     The plaintiffs claim that the Rule’s interpretation of
“single function” impermissibly shifts the statutory focus from
the trigger’s action to the trigger finger’s action. But the Rule
defines “single function” to mean “single pull of the trigger and
analogous motions.” The Rule’s definition describes the
“motion” of the trigger, not of the trigger finger. Id. at 66,554.
Indeed, nothing in the Rule’s definition refers to a shooter’s
finger or a volitional action. Id. The plaintiffs challenge the
Rule because the ATF determines therein that a bump stock
device allows the firearm to shoot more than one shot with only
a single pull. But that is a question of application, not
definition. As for the definition, I believe the Rule correctly
                                 16
reads “function” by focusing on how the trigger acts—that is,
through a pull.

                      C. “Automatically”

     The Bump Stock Rule defines “automatically” to mean “as
the result of a self-acting or self-regulating mechanism that
allows the firing of multiple rounds through a single pull of the
trigger.” Id. at 66,519. The plaintiffs challenge this definition
because it does not account for the additional physical input the
shooter must provide in the firing sequence to make a firearm
with a bump stock shoot more rapidly. That “pull plus” action,
they say, invalidly expands the statutory text: a “‘single
function of the trigger’ is the starting and the ending point of
[making] a firearm automatic.” Codrea Br. at 14. I agree. 12

      The Rule’s fatal flaw comes from its “adding to” the
statutory language in a way that is—at least to me—plainly
ultra vires. 1A Sutherland Statutory Construction § 31.02, at
521 (4th ed. 1985) (“The legislative act is the charter of the
administrative agency and administrative action beyond the
authority conferred by the statute is ultra vires.”); see Burnet v.
Marston, 57 F.2d 611, 612 (D.C. Cir. 1932) (“While the
[agency] was clothed with authority to promulgate regulations,
[it] was not authorized to add to or take from the plain language

    12
        A portion of the Bump Stock Rule’s definition of
“automatically” strikes me as unobjectionable. It adopts the phrase
“functioning as the result of a self-acting or self-regulating
mechanism” as a substitute for “automatically.” Bump-Stock-Type
Devices, 83 Fed Reg. at 66,554. It does so because dictionaries in
use at the time the 1934 Act was enacted defined “automatically”
that way. Id. at 66,519; see also Webster’s New International
Dictionary 187 (2d ed. 1934) (“automatic” means “[h]aving a self-
acting or self-regulating mechanism that performs a required act at a
predetermined point in an operation”).
                                 17
of the statute, for, ‘where the intent is plain, nothing is left to
construction.’” (quoting United States v. Fisher, 6 U.S. 358,
386 (1805))). “Automatically” cannot be read in isolation. On
the contrary, it is modified—that is, limited—by the clause “by
a single function of the trigger.” 26 U.S.C. § 5845(b);
Webster’s New International Dictionary 307 (2d ed. 1934)
(defining “by” as “through the means of”). Section 5845(b)’s
awkward syntax does not equal ambiguity, as illustrated by the
lost art of diagramming. 13 “Automatically . . . by a single
function of the trigger” is the sum total of the action necessary
to constitute a firearm a “machinegun.” 26 U.S.C. § 5845(b).
A “machinegun,” then, is a firearm that shoots more than one
round by a single trigger pull without manual reloading. 14 The
     13
          Section 5845(b) can be diagrammed as follows:




See generally Marye Hefty et al., Sentence Diagramming 7–11, 17–
20, 24–25, 30–31, 33, 49 (2008).
     14
        In United States v. Olofson, 563 F.3d 652 (7th Cir. 2009), the
Seventh Circuit discussed the meaning of “machinegun.” It
explained that “‘automatically’ is the adverbial form of ‘automatic,’”
meaning “[h]aving a self-acting or self-regulating mechanism.” Id.
at 658 (alteration in original) (quoting Webster’s New International
Dictionary 187 (2d ed. 1934)). It then read section 5845(b)’s
“automatically” as follows: “the adverb ‘automatically,’ as it
modifies the verb ‘shoots,’ delineates how the discharge of multiple
rounds from a weapon occurs: as the result of a self-acting
                                18
statutory definition of “machinegun” does not include a firearm
that shoots more than one round “automatically” by a single
pull of the trigger AND THEN SOME (that is, by “constant
forward pressure with the non-trigger hand”). Bump-Stock-
Type Devices, 83 Fed. Reg. at 66,532. By including more
action than a single trigger pull, the Rule invalidly expands
section 5845(b), as the ATF itself recognized in the
rulemaking. See id. (shooter “maintain[s] constant forward
pressure with the non-trigger hand on the barrel-shroud or fore-
grip of the rifle,” and “maintain[s] the trigger finger on the
device’s extension ledge with constant rearward pressure.”).

     My reading of the statute comports with the common sense
meaning of the language used. Suppose an advertisement
declares that a device performs a task “automatically by a push
of a button.” I would understand the phrase to mean pushing
the button activates whatever function the device performs. It
would come as a surprise, I submit, if the device does not
operate until the button is pushed and some other action is
taken—a pedal pressed, a dial turned and so on. Although the
device might be “automatic” under some definition, it would
not fit the advertised definition of “automatic”: by a push of a
button period.

    More importantly, my reading of the statute—unlike the
ATF’s reading—maintains the longstanding distinction
between “automatic” and “semiautomatic” in the firearms
context. The original definition of “machinegun” in the 1934
Act included a firearm that shoots more than one round

mechanism.” Id. My rejection of the Bump Stock Rule creates no
tension with Olofson. That court did not consider whether additional
manual input from the non-shooting hand—“pull plus”—takes a
device outside section 5845(b)’s definition of “automatically.” Nor
did Olofson consider whether “pull” refers to how the trigger works
or to the movement of the shooter’s trigger finger.
                                19
“automatically or semiautomatically.” 26 U.S.C. § 2733(b)
(1940). At the time, an “automatic gun” was understood to be
“[a] firearm which, after the first round is exploded, by gas
pressure or force of recoil automatically extracts and ejects the
empty case, loads another round into the chamber, fires, and
repeats the above cycle, until the ammunition in the feeding
mechanism is exhausted, or pressure on the trigger is released.”
Webster’s New International Dictionary 187 (2d ed. 1934). A
“semiautomatic gun” was (and is) “[a] firearm in which part,
but not all, of the operations involved in loading and firing are
performed automatically, as when the recoil is used to open the
breech and thus prepare for reloading by hand.” Webster’s
New International Dictionary 187 (2d ed. 1934). At the time
of the 1934 Act’s enactment, then, the difference between an
“automatic” and a “semiautomatic” gun depended on whether
the shooter played a manual role in the loading and firing
process. My interpretation fits the historical context by
limiting “automatic[]” to a firearm that shoots more than one
round by a single trigger pull with no additional action by the
shooter. By contrast, the Bump Stock Rule reinterprets
“automatically” to mean what “semiautomatically” did in
1934—a pull of the trigger plus. The Congress deleted
“semiautomatically” from the statute in 1968 and the ATF is
without authority to resurrect it by regulation.

     The ATF insists that my interpretation renders
“automatically” superfluous—a result inconsistent with the
well-established principle that “‘[a] statute should be construed
so that effect is given to all its provisions, so that no part will
be inoperative or superfluous, void or insignificant.’” Corley
v. United States, 556 U.S. 303, 314 (2009) (alteration in
original) (quoting Hibbs v. Winn, 542 U.S. 88, 101 (2004)).
Not even close. “[A]utomatically” means that the firearm
shoots more than one shot as the result of a self-acting
mechanism effected by a single pull of the trigger. Thus, the
                                20
combination of “automatically” and “by a single pull” explains
how the shooter accomplishes the firing sequence of a
“machinegun.” Under my reading, “automatically” excludes a
“machinegun” that uses a self-acting firing sequence effected
by action in addition to a single pull of the trigger.

     Finally, the ATF, as well as the district court, posits that
the Bump Stock Rule meets one ordinary meaning of
“automatically”—that is, “perform[s] parts of the work
formerly or usually done by hand.”             Webster’s New
International Dictionary 187 (2d ed. 1934). Both believe that
a bump stock “makes it easier to bump fire because it controls
the distance the firearm recoils and ensures that the firearm
moves linearly—two tasks the shooter would ordinarily have
to perform manually.” Guedes, 356 F. Supp. 3d at 132. Maybe
so. But the Rule does not use the “formerly done by hand”
meaning of “automatically.” Bump-Stock-Type Devices, 83
Fed. Reg. at 66,519. It defines “automatically” to mean “as the
result of a self-acting or self-regulating mechanism.” Id.
Whether that definition is consistent with section 5845(b)’s
definition is the question before us. 15

           D. Is a Bump Stock a “Machinegun?”

     Having interpreted “automatically” and “single function of
the trigger,” the Rule declares that a “‘machinegun’ includes a
bump-stock-type device, i.e., a device that allows a semi-
automatic firearm to shoot more than one shot with a single pull

    15
        I am not quibbling about semantics. The two definitions of
“automatically” have different aims: one refers to a self-acting
object; the other refers to automating a formerly “by-hand” task.
Webster’s Third New International Dictionary 148 (1993). The
“formerly by-hand” definition would shift the focus from whether a
bump stock provides a self-acting mechanism to fire multiple rounds
to whether a bump stock automates any action in the firing sequence.
                                21
of the trigger by harnessing the recoil energy of the semi-
automatic firearm to which it is affixed so that the trigger resets
and continues firing without additional physical manipulation
of the trigger by the shooter.” Id. at 66,553–54. There are at
least two defects in this classification. It ignores the fact that a
non-mechanical bump stock—a type of bump stock device
covered by the Rule—does not allow the firearm to shoot more
rapidly with a single pull of the trigger because the shooter
must provide “constant forward pressure with the non-trigger
hand” for the device to function. Id. at 66,532. It also
erroneously determines that a bump stock allows a
semiautomatic rifle to fire more than one round with a single
pull of the trigger. For these reasons, I agree with the plaintiffs
that a bump stock is not a “machinegun.”

     First, a firearm equipped with a non-mechanical bump
stock does not fire “automatically” because the shooter must
also provide constant forward pressure with his non-shooting
hand. The Rule’s very description of a non-mechanical bump
stock manifests that its proscription is ultra vires:

        [Bump stock] devices replace a rifle’s standard
        stock and free the weapon to slide back and
        forth rapidly, harnessing the energy from the
        firearm’s recoil either through a mechanism like
        an internal spring or in conjunction with the
        shooter’s maintenance of pressure (typically
        constant forward pressure with the non-trigger
        hand on the barrel-shroud or fore-grip of the
        rifle, and constant rearward pressure on the
        device’s extension ledge with the shooter’s
        trigger finger).

Id. at 66,516 (emphases added). This description covers two
types of bump stocks, one that includes a mechanism like an
                                 22
internal spring and the other that requires the shooter to
maintain pressure with his non-trigger hand. Id. The first type,
including the original Akins Accelerator, has been classified as
a “machinegun” and hence illegal since 2006. Id. at 66,517.
The Rule must—and does—aim at the second type—the non-
mechanical bump stock—which operates only in conjunction
with the shooter’s added physical pressure. 16 But that added
physical pressure is inconsistent with the statutory definition of
a “machinegun,” which fires multiple rounds with a self-acting
mechanism effected through a single pull of the trigger
simpliciter. In short, the statute uses “pull” and the Rule—
invalidly—uses “pull plus.”

     Other parts of the Rule expose the ATF’s error. In
discussing its interpretation of “automatically,” the ATF gave
the following explanation: “[s]o long as the firearm is capable
of producing multiple rounds with a single pull of the trigger
until [1] the trigger finger is removed, [2] the ammunition
supply is exhausted, or [3] the firearm malfunctions, the
firearm shoots ‘automatically’ irrespective of why the firing
sequence ultimately ends.” Id. at 66,519. Yet elsewhere the
ATF describes the firing process of a firearm with a bump stock
as follows: “the shooter ‘pulls’ the trigger once and allows the
firearm and attached bump-stock-type device to operate until
the shooter releases the trigger finger or the constant forward
pressure with the non-trigger hand.” Id. at 66,532 (emphasis
added). In my view, this assertion is an explicit recognition
that a bump stock device does not continue shooting rounds
with a single trigger pull if the shooter does not maintain

     16
        At oral argument, the ATF asserted that the non-trigger
hand’s “additional forward pressure” is part of the “automatic” firing
process. Transcript of Oral Argument 73–74. “Automatic” means
“self-acting or self-regulating.” Bump-Stock-Type Devices, 83 Fed.
Reg. at 66,553. The non-trigger hand’s constant forward pressure
requires physical, not automatic, action.
                              23
“constant forward pressure with the non-trigger hand.” Id. at
66,532.

     Moreover, I find it difficult to ignore the ATF’s repeated
earlier determinations that non-mechanical bump stocks do not
initiate an automatic firing sequence.             Three ATF
determination letters from 2010 to 2013 explained why non-
mechanical bump stocks are not “machineguns”:

       [Our] evaluation confirmed that the submitted
       stock (see enclosed photos) does attach to the
       rear of an AR-15 type rifle which has been fitted
       with a sliding shoulder-stock type buffer-tube
       assembly. The stock has no automatically
       functioning mechanical parts or springs and
       performs no automatic mechanical function
       when installed. In order to use the installed
       device, the shooter must apply constant forward
       pressure with the non-shooting hand and
       constant rearward pressure with the shooting
       hand.

Determination Letter signed by John R. Spencer, Chief,
Firearms Tech. Branch, ATF (June 7, 2010), reprinted at J.A.
278; see also Determination Letter signed by John R. Spencer,
Chief, Firearms Tech. Branch, ATF (April 2, 2012), reprinted
at J.A. at 279–80; Letter from Richard W. Marianos, Assistant
Dir. Pub. and Governmental Affairs, to Congressman Ed
Perlmutter (April 16, 2013), reprinted at J.A. 281–82. The
Rule does not fairly treat the ATF’s repeated determinations
that a non-mechanical bump stock “performs no automatic
mechanical function when installed.” J.A. 278. Instead, it
rejects its previous reading as based on an incomplete legal
definition of “automatically.” Bump-Stock-Type Devices, 83
                                  24
Fed. Reg. at 66,521. 17 But those determinations made factual
findings that the non-mechanical bump stock operates only if
the shooter applies “constant forward pressure with the non-
shooting hand and constant rearward pressure with the shooting
hand.” Determination Letter signed by John R. Spencer, Chief,
Firearms Tech. Branch, ATF (June 7, 2010). And those factual
findings dictate that a non-mechanical bump stock is not a
“machinegun” under section 5845(b).

     Second, a semiautomatic rifle equipped with a bump stock
cannot fire more than one round with a single function of the
trigger. The plaintiffs argue—and the ATF does not dispute—
that the trigger of a semiautomatic rifle must release the
hammer for each individual discharge. Nor is there any dispute
that a semiautomatic rifle cannot fire again until the trigger is
released, which causes the hammer to reset. The Rule refers to
the release of the trigger as a “separate” function. Bump-Stock-
Type Devices, 83 Fed. Reg. at 66,534 (“While semiautomatic
firearms [equipped with certain devices] may shoot one round
when the trigger is pulled, the shooter must release the trigger
before another round is fired. Even if this release results in a
second shot being fired, it is as the result of a separate function
of the trigger.”). Once the trigger shoots, it must be released to
reset the hammer and the trigger must be pulled again for each
subsequent shot. Verified Declaration of Richard (Rick)

     17
         During the rulemaking, the ATF repeatedly declared that its
earlier determinations “did not include extensive legal analysis of the
statutory terms ‘automatically’ or ‘single function of the trigger.’”
Bump-Stock-Type Devices, 83 Fed. Reg. at 66,516; see also id. at
66,514, 66,521, 66,528, 66,531. I defy a careful reader of the
rulemaking to find any legal, as opposed to functional, analysis of a
bump stock device, much less substantial legal analysis. Id. at
66,518 (“[P]rior ATF rulings concerning bump-stock-type devices
did not provide substantial or consistent legal analysis regarding the
meaning of the term ‘automatically.”’).
                                 25
Vasquez, former Acting Chief of the Firearms Tech. Branch of
ATF, at 4 (with bump stock, “after the first shot is discharged,
the trigger must be released, reset, and pulled completely
rearward, before the subsequent round is discharged”),
reprinted at J.A. 275. Thus, a semiautomatic rifle equipped
with a bump stock cannot shoot more than one round with a
single pull of the trigger. 18

      Still, the ATF insists that a bump stock allows a firearm to
shoot multiple shots with a single pull. Bump-Stock-Type
Devices, 83 Fed. Reg. at 66,553–54. The ATF focuses on
whether the shooter must pull his index finger more than once
to fire multiple shots. Because a bump stock allows the firearm
to fire more than once with a single pull of the index finger, the
ATF concludes that a bump stock is a “machinegun.”
Remember, however, section 5845(b) uses “single function of
the trigger,” not single function of the shooter’s trigger finger.

    If the focus is—as it must be—on the trigger, a bump stock
does not qualify as a “machinegun.” A semiautomatic rifle
shoots a single round per pull of the trigger and the bump stock
changes only how the pull is accomplished. Without a bump
stock, the shooter pulls the trigger with his finger for each shot.
With a bump stock, however, the shooter—after the initial
pull—maintains backward pressure on the trigger and puts

    18
        Record evidence supports my point. As discussed earlier, the
record includes a video of a shooter firing a rifle equipped with a
bump stock. The video is in slow motion and focuses on the trigger.
For each shot the rifle fires, the trigger is pulled by the shooter’s
stationary trigger finger. The trigger is then released between each
shot. And the trigger is pulled again for the next shot. This trigger
movement confirms that a bump stock does not allow a rifle to shoot
more than one round with only a single pull of the trigger. Attached
as an appendix are photographs, taken from the video, that illustrate
the trigger’s movement during the bump stock’s firing sequence.
                               26
forward pressure on the barrel with his non-shooting hand;
these manual inputs cause the rifle to slide and result in the
shooter’s stationary finger pulling the trigger. Bump-Stock-
Type Devices, 83 Fed. Reg. at 66,533 (“The constant forward
pressure with the non-trigger hand pushes the firearm forward,
again pulling the firearm forward, engaging the trigger, and
firing a second round.”). The bump stock therefore affects
whether the shooter pulls his trigger finger or keeps it
stationary. It does not change the movement of the trigger
itself, which “must be released, reset, and fully pulled rearward
before [a] subsequent round can be fired.”                Verified
Declaration of Richard (Rick) Vasquez, former Acting Chief
of the Firearms Tech. Branch of ATF, at 3–4.

     Like countless other Americans, I can think of little
legitimate use for a bump stock. That thought, however, has
nothing to do with the legality of the Bump Stock Rule. For
the reasons detailed supra, I believe the Bump Stock Rule
expands the statutory definition of “machinegun” and is
therefore ultra vires. In my view, the plaintiffs are likely to
succeed on the merits of their challenge and I would grant them
preliminary injunctive relief.

    Accordingly, I respectfully dissent.
                           27
                        APPENDIX

Photograph One: Trigger separates from stationary index
finger.




Photograph Two: Trigger comes into contact with stationary
index finger.
