                              In the

    United States Court of Appeals
                For the Seventh Circuit
                    ____________________
No. 14-3602
UNITED STATES OF AMERICA,
                                                  Plaintiff-Appellee,

                                v.

JOSHUA R. MACKIN,
                                              Defendant-Appellant.
                    ____________________

        Appeal from the United States District Court for the
        Northern District of Indiana, Fort Wayne Division.
        No. 1:13-cr-00062 — Theresa L. Springmann, Judge.
                    ____________________

       ARGUED MAY 28, 2015 — DECIDED JULY 13, 2015
                    ____________________

   Before FLAUM, KANNE, and SYKES, Circuit Judges.
    KANNE, Circuit Judge. Appellant Joshua Mackin was
charged with one count of being a felon in possession of a
firearm. During discovery, the government disclosed an in-
complete copy of the continuity slip used to track the fire-
arm’s chain of custody. Later, at trial, the government pro-
duced the correct and complete copy. Mackin moved for a
mistrial, arguing that the government violated its Rule 16
obligation to turn over the correct and complete document,
2                                                No. 14-3602

as the document was material to preparing his defense. The
trial court denied his motion, and he now appeals that deni-
al. For the reasons discussed below, we vacate Mackin’s con-
viction and remand the case for further proceedings.
                        I. BACKGROUND
    On August 2, 2013, Fort Wayne police officers Treven
Brown and Alexander Ramon went to the home of Appellant
Joshua Mackin, seeking to serve him with a valid arrest war-
rant. The officers apprehended Mackin in the alley outside
his home. After placing Mackin in handcuffs, Officer Ramon
searched Mackin’s person. From Mackin’s pants pocket, Of-
ficer Ramon recovered a silver handgun that was loaded
with five rounds. Officer Brown confiscated the firearm, and
he recorded its serial number on an incident report. Officer
Brown testified that the weapon was within his view or pos-
session from the time Officer Ramon recovered it until Of-
ficer Brown secured it in an evidence locker.
    At the time he secured it in the evidence locker, Officer
Brown completed a “continuity slip” for the firearm. A con-
tinuity slip is a document designed to track the movements
of a piece of evidence while it remains in law enforcement’s
custody. Officer Brown testified that, adhering to protocol,
he filled out and signed the continuity slip at the time he
placed the firearm in the evidence locker. He then placed the
continuity slip in the locker with the firearm.
    One month after Mackin’s arrest, Bureau of Alcohol, To-
bacco, Firearms and Explosives (“ATF”) Special Agent Craig
Edwards examined a firearm bearing the same serial number
as the number listed on the incident report prepared by Of-
No. 14-3602                                                    3

ficer Brown. Agent Edwards determined that the weapon
was functional and was manufactured in California.
    On August 28, 2013, a grand jury returned a one-count
indictment against Mackin, charging him with being a felon
in possession of a firearm in violation of 18 U.S.C.
§ 922(g)(1). In an accompanying allegation, the government
sought the forfeiture of the firearm, identifying it by the seri-
al number “3224934.” The district court set Mackin’s trial for
August 19, 2014.
    At some point prior to trial, the government turned over
discovery materials requested by Mackin. Included in those
materials was a document entitled “F.W.P.D. Continuity
Record,” or the continuity slip discussed above. The slip is a
standard form that includes a variety of labeled boxes, or
fields, into which one enters information about items of evi-
dence. Several fields are designed to track the movement of a
piece of evidence, including a series of signature boxes
marked as “From: Signature, PE” and “To: Signature, PE.”
    The continuity slip produced to Mackin before trial con-
tained a number of empty fields. To be sure, it included in
typewritten form Mackin’s name, the date, time, and address
of the “original investigation,” and a description of the fire-
arm allegedly obtained from Mackin. In the first “From”
signature box, the word “copy” is handwritten, and in the
first “To” signature box, “Locker #26” is handwritten. But
the remaining “From” and “To” boxes are blank, and no sig-
natures appear anywhere on the continuity slip. It was in-
complete.
    On August 5, 2014, the government filed a motion to cor-
rect what it called a “scrivener’s error” in the forfeiture alle-
4                                                         No. 14-3602

gation that had accompanied Mackin’s indictment. The mo-
tion to correct indicated that in the forfeiture document, the
firearm’s serial number did not match the serial numbers
listed in Officer Brown’s report, the ATF’s search record, or
the serial number inscribed on the firearm itself. 1 The district
court granted the motion, and the government corrected the
serial number as listed in the forfeiture allegation. The gov-
ernment did not correct or supplement the incomplete con-
tinuity slip.
    On August 12, 2014, one week before trial, Mackin filed a
proposed jury instruction involving a duress defense. The
government responded with a motion in limine, requesting
that Mackin be required to establish a prima facie case of du-
ress in order to present that affirmative defense instruction
to the jury. The district court denied Mackin’s proposed jury
instruction and granted the government’s motion. That deci-
sion left Mackin with only one line of defense: the largely
incomplete continuity slip.
    At trial, the government called Officer Brown as its first
witness. Officer Brown testified to the circumstances of
Mackin’s arrest, including the recovery of the firearm. Dur-
ing that testimony, the government provided Officer Brown
with his incident report and a copy of the continuity slip that
had been disclosed to the defense. After refreshing his recol-
lection with those documents, Officer Brown testified that
the serial number of the weapon he recovered from Mackin
was 3224394. The government then showed Officer Brown a


1 The forfeiture document listed the serial number as 3224934. The fire-
arm’s actual serial number, and the number recorded on all other docu-
ments, was 3224394.
No. 14-3602                                                5

firearm marked as “Government Exhibit 1.” Officer Brown
testified that the gun appeared to be the same one that he
recovered from Mackin. After comparing the serial numbers,
he testified that it was indeed the same firearm. The gov-
ernment then moved to admit the firearm into evidence.
    Mackin objected to the firearm’s admission and was
granted leave to voir dire Officer Brown regarding the exhib-
it. Officer Brown described the normal protocol for securing
a weapon, storing the evidence, and completing a continuity
slip. He stated that, as the officer placing the weapon into
storage, he completed and signed the continuity slip that ac-
companied the weapon. The slip was then placed in the evi-
dence locker with the firearm. Any person who later ac-
cessed the weapon was then required to sign the form, so
that, as Officer Brown explained, “we know who has that
weapon and where it’s at at all times.”
    Mackin asked Officer Brown to point out where his sig-
nature appeared on the continuity form that had been pro-
duced to Mackin in discovery. Officer Brown acknowledged
that his signature did not appear anywhere on the docu-
ment, but stated that “if [defense counsel had] the original,
[Officer Brown] would be more than happy to show [coun-
sel] on that.” Defense counsel, however, had only the in-
complete copy provided during discovery. The court then
asked the government if the original form was available, to
which the government responded that it would check its
files.
   Following that voir dire, Mackin again objected to the
admission of the firearm. He argued that the government
could not lay the proper foundation for its admission into
evidence because there were no signatures or other notations
6                                                 No. 14-3602

on the form to document the weapon’s handling while in law
enforcement’s possession. Without that, he argued, the gov-
ernment could not establish the gun’s chain of custody. He
stated that “apparently there is an entirely separate docu-
ment that’s been prepared by Officer Brown that Mr. Mackin
has not been provided as part of his defense.”
    The court then held a bench conference with counsel out-
side the hearing of the jury. During the conference, the case
agent seated with the government produced a continuity
slip that differed from the one Mackin had. This new form
contained a number of signatures in the “To” and “From”
fields, continuing onto a second page, as well as an indica-
tion that the firearm was at one point located in the “ATF
vault.” The government conceded that this new form had
additional information that was not present on the previous-
ly disclosed form. The government also conceded that it had
not previously disclosed this completed form to Mackin.
    Mackin objected to the introduction of the new continui-
ty form and then moved for a mistrial. He stated that the is-
sue of continuity—and the related chain of custody—was “in
large part” his defense. Mackin argued that he was preju-
diced by the government’s nondisclosure in a manner from
which he could not recover. According to Mackin’s counsel,
had the defense been given the correct continuity slip, “we
may not even be here today, because Mr. Mackin may have
very well pled.”
    The court denied Mackin’s mistrial motion and allowed
both continuity slips to be presented to the jury. It also al-
lowed Mackin to cross-examine the relevant witnesses re-
garding the two different continuity slips. The firearm was
ultimately admitted into evidence, and the jury found
No. 14-3602                                                   7

Mackin guilty of one count of being a felon in possession of a
firearm.
                          II. ANALYSIS
    On appeal, Mackin argues that the government violated
Federal Rule of Criminal Procedure 16 by failing to disclose
the correct continuity slip. Mackin argues that he was preju-
diced by that violation, and that the district court abused its
discretion in denying his motion for a mistrial. For the rea-
sons discussed below, we conclude that the government did
violate Rule 16 and that mistrial was the appropriate remedy
to cure that violation.
   A. Rule 16 Violation
    Rule 16 governs the discovery and inspection of evidence
in criminal matters. It provides in relevant part that:
   Upon a defendant’s request, the government must
   permit the defendant to inspect and to copy or photo-
   graph books, papers, documents, data, photographs,
   tangible objects, buildings or places, or copies or por-
   tions of any of these items, if the item is within the
   government’s possession, custody, or control and:
   (i) the item is material to preparing the defense;
   (ii) the government intends to use the item in its case-
   in-chief at trial; or
   (iii) the item was obtained from or belongs to the de-
   fendant.
Fed. R. Crim. P. 16(a)(1)(E). The rule also imposes a con-
tinuing duty of disclosure, should a party uncover addi-
tional discoverable material prior to or during trial. Fed.
R. Crim. P. 16(c).
8                                                  No. 14-3602

    Mackin argues that an accurate copy of the continuity
slip was material to preparing his defense, and therefore the
government violated Rule 16(a)(1)(E)(i) by failing to disclose
it before trial. The government responds that it did not
commit a violation of Rule 16, because it “had no reason to
suspect that Mackin would contest chain of custody or that
continuity slips would be material to Mackin’s defense.” We
agree with Mackin that a Rule 16 violation occurred in this
case.
    It is unclear whether the government concedes that it
erred in disclosing the unsigned, nearly blank continuity
slip. Its comments to the district court seem to suggest that it
did err either in disclosing the wrong version of the form, or
in failing to supplement its discovery. See, for example, the
following exchange between defense counsel and the gov-
ernment during the court’s sidebar discussion:
    [Defense counsel]: So we get this document … now
    in the middle of trial, they’re saying “oh, no, no, no,
    there’s a continuity report, don’t worry about it. This
    is the same gun.” We didn’t know that until about 2
    minutes ago.
    [Prosecutor]: Well, Your Honor, the Government pro-
    vided…
    [Defense counsel]: That’s a totally different docu-
    ment.
    [Prosecutor]: The government provided the continui-
    ty form we had.
    The Court: But never updated or supplemented with
    the one you have in hand.
No. 14-3602                                                    9

   [Prosecutor]: I’m sorry, I do have that now.
The government has not explained why it did not produce
the complete continuity slip before trial, but it argues on ap-
peal that it had no obligation to do so in any event. The gov-
ernment contends that Rule 16 did not require it to produce
any continuity slip—complete or incomplete—in the first
place because there was no reason to suspect that the slip
would be material to Mackin’s defense.
    First, we note that the government never argued at trial
that the continuity slip was beyond the reach of discovery
under Rule 16; on the contrary, it produced the incomplete
slip during discovery and then affirmatively sought to intro-
duce the correct form at trial—either as helpful for its case-
in-chief or to rebut Mackin’s defense. In addition, we ques-
tion the government’s conclusory statement that it had no
reason to believe the gun’s chain of custody would be mate-
rial to the defense.
   But, putting those issues to the side, the government
misses the critical issue. Mackin does not argue that he had
an absolute right to disclosure of the continuity slip, or that
continuity slips are mandatory discovery material in the
mine-run of cases. Instead, he argues that in this particular
case, once the government chose to disclose the incomplete
continuity slip, he was entitled to the complete and correct one.
We agree, and we hold as much today.
    Mackin received a continuity slip that was devoid of the
signatures that he knew were required. The government’s
own witness testified to the importance of the continuity slip
in safeguarding the firearm as it remained in law enforce-
ment custody. Indeed, as far as we can tell, that is the entire
10                                                 No. 14-3602

purpose of a continuity slip. The signatures, as well as the
descriptions of the firearm’s locations, were the only way to
track the firearm’s movements. Mackin was justified in be-
lieving that the continuity slip would be used for its obvious
purpose—the purpose verified by law enforcement officials
at trial. The slip he was given was clearly deficient, and it
reasonably led him to suspect that the government had not
followed the proper chain-of-custody procedures in his case.
    This, in turn, led him to think he had a plausible defense
based on the incomplete continuity slip. Indeed, after the
district court denied his motion to introduce the duress de-
fense, the continuity slip was Mackin’s only line of defense
against nearly certain conviction. As it turned out, however,
this defense was not plausible: there was another, complete
form that fully tracked the firearm’s chain of custody. The
problem is, he did not know that until the government pro-
duced the new form at trial.
    We acknowledge the possibility that continuity slips may
not always be discoverable. But once the government chose
to disclose the incomplete slip, the defense was entitled to
rely on that disclosure as being an accurate representation of
the evidence the government had in its possession. See Unit-
ed States v. Lee, 573 F.3d 155, 161 (3d Cir. 2009) (stating that
“Rule 16’s discovery requirements made it reasonable for de-
fense counsel to rely on the Government to provide, in the
first instance, an accurate copy of the evidence offered at tri-
al.”). The defense was also entitled to assume that the gov-
ernment would supplement its discovery if necessary, as re-
quired by Rule 16(c).
  As Mackin’s counsel expressed at oral argument, Mackin
would have been better off in this case had the government
No. 14-3602                                                 11

not disclosed a continuity slip at all. The government’s own
error led Mackin to believe he had a viable defense. In the
circumstances of this case, the burden of that error should
fall on the government.
    Further, even assuming the government initially had no
reason to predict a chain-of-custody defense, it certainly had
a reason to predict it after disclosing the incomplete continu-
ity slip. 18 U.S.C. § 922(g)(1) makes it unlawful for any per-
son “who has been convicted in any court of a crime punish-
able by imprisonment for a term exceeding one year … to
possess in or affecting commerce, any firearm or ammunition;
or to receive any firearm or ammunition which has been
shipped or transported in interstate or foreign commerce.” 18
U.S.C. § 922(g)(1) (emphasis added).
    Because Mackin had stipulated to the prior felony convic-
tion, only his possession of the firearm and its nexus to in-
terstate commerce remained as triable issues. Proof of the
firearm’s chain of custody went directly to the weight the ju-
ry could assign the firearm evidence—including the ATF’s
assessment of its link to interstate commerce. This is in addi-
tion to the fact that the government had already discovered a
“scrivener’s error” involving the serial number of the fire-
arm—another mistake that tends to cast doubt on the gov-
ernment’s handling of the evidence. The government was
sufficiently on notice after its incomplete disclosure to have
anticipated the chain-of-custody defense, and at that point, it
should have supplemented its disclosure under Rule 16(c)
by producing the complete continuity slip.
12                                                No. 14-3602

     B. Mistrial Motion and Prejudice
    We review the denial of a Rule 16(d)(2) motion for an
abuse of discretion. United States v. De La Rosa, 196 F.3d 712,
715 (7th Cir. 1999). In addition, we will not disturb a ruling
on a Rule 16 sanctions motion absent a showing of prejudice.
Id. at 715-16.
    It is firmly established that a defendant is prejudiced un-
der Rule 16 when he is “unduly surprised and lacks an ade-
quate opportunity to prepare a defense.” Id. at 716. Mackin
argues that he was denied an adequate opportunity to pre-
pare a defense because he formed his trial strategy relying
on the incomplete evidence that had been provided by the
government. The government argues that Mackin did not
suffer any prejudice. We agree with Mackin that he suffered
prejudice under the particular facts of this case—specifically
relating to the interstate movement of the firearm.
    As previously described, Mackin stipulated to the fact
that he had been convicted of a qualifying felony. So the
government, in order to secure a conviction, had to prove
two elements: (1) that Mackin possessed a firearm; and (2)
that the firearm had been possessed in or affecting com-
merce, or that the firearm had been shipped or transported
in interstate or foreign commerce.
    The government’s evidence regarding his possession of a
firearm was strong. So it appears to us that the government’s
primary burden at trial was to prove the firearm’s link to in-
terstate commerce. (That nexus is, after all, what makes this
a federal offense.) The government would seek to prove that
element, as is the case in most felon-in-possession cases,
through the ATF’s analysis of the weapon’s manufacture and
No. 14-3602                                                  13

sale. Mackin, for his part, then had to undermine the gun’s
link to interstate commerce. He sought to do so by casting
doubt on law enforcement’s handling of the evidence, and in
particular, its storage and handling by the Fort Wayne Police
Department and the ATF. By pointing out gaps in the weap-
on’s chain of custody, Mackin could alter the weight as-
signed to the firearm evidence. See United States v. Tatum, 548
F.3d 584, 587 (7th Cir. 2008). That includes the weight as-
signed to the firearm exhibit itself, if admitted into evidence,
as well as potential challenges to the ATF’s analysis of the
firearm.
    Mackin argues that had the government disclosed the
correct continuity form, he would have known that his de-
fense strategy was implausible. Had the government dis-
closed the completed form, he contends, he would have
known that the weapon’s chain of custody had been correct-
ly documented, and he would have had no basis upon which
to raise a chain-of-custody defense. Because the govern-
ment’s own error raised the very possibility of the defense he
attempted to assert at trial, Mackin argues, the Rule 16 viola-
tion “unduly surprised” him and deprived him of the “op-
portunity to prepare a defense.” Id.
    We find instructive the case of United States v. Lee. 573
F.3d 155. The appellant there was charged with possessing a
quantity of crack cocaine that had been found in a hotel
room on January 7, 2005. Id. at 158-59. During discovery, the
government disclosed to Lee a photocopy of the front side of
the hotel room’s registration card. Id. at 159. The photocop-
ied portion indicated that an individual named “Omar Mar-
tin,” who shared Lee’s home address, checked in on January
3 for a one-night stay. Id. at 158.
14                                                   No. 14-3602

   Lee argued at trial, based on the registration card, that
“Martin” (presumably Lee) had checked out of the hotel
room days before the drugs were found. Id. at 159. There-
fore, Martin/Lee could not be proven to have possessed the
drugs. When the case went to the jury, the jurors were pro-
vided with the trial exhibits for their deliberations. Id. at 160.
These exhibits included the original registration card. Id.
    Several hours after beginning to deliberate, the jury
asked the court, “What is the information on the back of the
registration card in pencil?” Id. Until that time, neither the
prosecution nor the defense was aware that the back of the
registration card contained additional notations. Id. The back
of the card contained dates (written by an unidentified per-
son) indicating that “Martin” had extended his stay at the
hotel through January 7—the day that the drugs were found.
Id. at 158.
    Lee moved for a mistrial, since the prosecution had failed
to disclose the information on the back of the card prior to
trial. Id. at 160. The judge denied the motion, instead in-
structing the jury to ignore the notations on the back of the
card. Id. Lee was ultimately convicted.
    On appeal, the Third Circuit held that Lee had been prej-
udiced by the government’s nondisclosure and that mistrial
was the appropriate remedy. Id. at 164-65. To begin, the court
noted that the information contained on the back of the card
would “entirely defeat Lee’s defense argument.” Id. at 161.
The court concluded that Lee “was deprived of any oppor-
tunity to prepare meaningfully for trial [or] to design an in-
telligent trial strategy.” Id. at 165. The court held that:
No. 14-3602                                                    15

      [a]bsent the discovery violation, Lee would have like-
      ly crafted a different trial strategy that might have
      proven more effective in light of the information on
      the back of the registration card. Finally, Lee might
      have chosen to enter into plea negotiations with the
      Government if he had accurate information about the
      strength of its case.
Id.
    So it is with Mackin’s case. The incomplete continuity
form led Mackin to believe that he had a viable chain-of-
custody defense. Had the government disclosed the correct
form, Mackin would have known that he was wrong. In-
stead, he proceeded to trial believing that he could show that
the proper evidence-handling procedures had not been fol-
lowed, because that is what the evidence disclosed by the
government itself suggested. He developed his trial strategy
based on the information provided by the government—
information that proved, mid-trial, to be at best incomplete.
    The government argues that Mackin was not prejudiced
by this belated disclosure because he could have changed his
strategy mid-trial. According to the government, Mackin’s
opening statements “did not reference continuity or chain of
custody and did not commit him to any particular line of de-
fense.” But, as Mackin argues (without any rebuttal from the
government), this was his only possible line of defense—
however weak it may have been. This is especially true be-
cause the court had already denied his only other defense
strategy: his proposed jury instruction regarding duress.
    Mackin did his best to “plow through,” as the trial court
told him to do, after the disclosure. He cross-examined the
16                                                 No. 14-3602

witnesses concerning the two versions of the continuity slip.
But “plowing through” was not enough to cure the damage
caused by the government’s nondisclosure. Mackin’s whole
strategy going into trial was based on the absence of a com-
plete and correct continuity form. Once the government be-
latedly produced a complete form, no amount of cross-
examination could have helped Mackin. See United States v.
Noe, 821 F.2d 604, 607 (11th Cir. 1987) (holding that the gov-
ernment’s failure to turn over discoverable evidence “is so
serious a detriment to the preparation for trial and the de-
fense of serious criminal charges that where it is apparent, as
here, that the defense strategy may have been determined by
the failure to disclose, there should be a new trial.” (quoting
United States v. Rodriguez, 799 F.2d 649, 651 (11th Cir. 1986)
(per curiam))).
    Finally, Mackin’s counsel made clear on the record at trial
that the Rule 16 violation also impacted his ability to intelli-
gently consider his plea options. His counsel immediately
stated, upon the government’s introduction of the correct
form:
     The problem with them giving this to us now is our
     defense in large part is based upon the fact that there
     are problems with the continuity of this weapon.
     Now, if we had that report, with signatures on it, we
     certainly would have changed our approach to this. In
     fact, we may not even be here today, because Mr.
     Mackin may very well have pled.
    Adequate preparation of a defense includes the ability to
intelligently weigh one’s plea options. The denial of that op-
portunity here constituted prejudice. See United States v. Mu-
niz-Jaquez, 718 F.3d 1180 (9th Cir. 2013) (“A defendant who
No. 14-3602                                                    17

knows that the government has evidence that renders his
planned defense useless can alter his trial strategy. Or he can
seek a plea agreement instead of going to trial.”); Lee, 573
F.3d at 165 (“Lee might have chosen to enter into plea nego-
tiations with the Government if he had accurate information
about the strength of its case.”); United States v. Alvarez, 987
F.2d 77, 85 (1st Cir. 1993) (finding prejudice where defendant
was “deprived of opportunity to design an intelligent …
plea strategy”); United States v. Pascual, 606 F.2d 561, 565 (5th
Cir. 1979) (“It would be hard to make an argument with any
degree of plausibility that the use of this [evidence] without
prior production did not seriously prejudice the defendants
in exercising their option to plead not guilty.”); see also Lafler
v. Cooper, 132 S.Ct. 1376, 1381 (2012) (stating that “criminal
justice today is for the most part a system of pleas, not a sys-
tem of trials”).
    The government counters by arguing that if the belatedly
disclosed continuity form “had dramatically shifted the evi-
dence, Mackin was perfectly free to change his plea to guilty
in the middle of trial.” This argument ignores, of course, that
Mackin had by then almost certainly lost any benefit of a
guilty plea. Prior to trial, even if the government had chosen
not to offer Mackin a favorable plea agreement, he still could
have received the benefit of a two-point offense-level reduc-
tion for acceptance of responsibility. See U.S.S.G. § 3E1.1.
That likely would have resulted in at least a fifteen-month
reduction in his sentence. After trial began, however, the
benefit of that guidelines section was likely lost to Mackin:
the reduction typically does not apply to someone who
“puts the government to its burden of proof at trial.”
U.S.S.G. § 3E1.1, cmt. 2.
18                                                No. 14-3602

                        III. CONCLUSION
    For the reasons discussed above, we find that the district
court abused its discretion in denying Mackin’s motion for a
mistrial, and that Mackin was prejudiced by that denial. Ac-
cordingly, Mackin’s conviction is VACATED and we
REMAND this case for further proceedings consistent with
this opinion.
