PUBLISH
UNITED STATES COURT OF APPEALS
FOR THE TENTH CIRCUIT

________________ _________________ UNITED STATES OF AMERICA, Plaintiff - Appellee,

v. SCOTT RAY BISHOP, Defendant - Appellant. No. 18-4088

________________ _________________ Appeal from the United States District Court for the District of Utah (D.C. No. 2:16-CR-00662-DB-1)

________________ _________________ Jessica Stengel, Assistant Fede ral Public Defender (Scott Keith Wilson, Interim Federal Public Defender, with her on the briefs), Office of the Federal Public Defender for the District of Utah, Salt Lake Cit y, Utah, appearing for Appellant

. Ryan D. Tenney, Assistant United States Attorney (John W. Huber , United States Attorney, with him o n the brief), Office of the United States A ttorney for the District of Utah, Salt Lake City, Uta h, appearing for Appellee.

________________ _________________ Before BRISCOE , HOLMES , and McHUGH , Circuit Judges.

________________ _________________ BRISCOE , Circuit Judge.

________________ _________________ This is a direct criminal appeal from Defendant Scott Bishop’s convictions on one count of unlawfully manufactur ing machineguns, in violation of 26 U.S.C. § 5861(a), and one count of unlawfu lly possessing or transferri ng machineguns, in FILED United States Court of Appeals Tenth Circuit 

### June 10, 2019
Elisabeth A. Shumaker Clerk of Court2 violation of 18 U.S. C. § 922(o). A machine gun is “any weapon w hich shoots, is designed to shoot, or can be read ily restored to shoot, automat ically more than one shot, without manual reloading, by a single function of the tri gger.” 26 U.S.C. § 5845(b). “The term . . . also i nclude[s] . . . any part desi gned and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun . . . .” Id. Defendant designed and manufactured a device, referred to as a TCGTR, t hat he intended his customers to install in their AR-15 semiautomatic rifles to in crease the speed at which their guns fired. The government offered evidence that the TCGTR was a machinegun bec ause it increased an AR-15’s rate of fire by causi ng the gun to fire multiple bul lets per pull of the trigger. Defendant, who proceeded pro se a t trial, took th e stand in his own defense and testified that he did not int end the TCGTR to convert an AR -15 into a machinegun. The district court excluded portions of Defendant’ s testimony after finding that it was expert testimo ny not properly disclosed to the government. Defendant, now represented by co unsel on appeal, argues that th e jury’s verdict should be set aside because the d istrict court denied him his c onstitutional right to present a defense, erred when in structing the jury on the eleme nts of a § 5861(a) offense, improperly admitted h earsay testimony about the legali ty of the TCGTR, and allowed unsupported expert testimony on an ultimate issue of fa ct. Exercising jurisdiction pursuant to 28 U. S.C. § 1291, we affirm.3 I In 2015, the Bureau of Alcohol, T obacco, Firearms and Explosive s recovered a “machine gun conversion device” wh ile executing a search warran t in an unrelated case. App. Vol. III at 77. The d evice “had pap erwork with it that explained how it worked and it had pictures on it that showed how [to] install t h[e] device.” Id. The paperwork indicated that the de vice was sold by a company calle d “TCGTR” through a website called “arfakit.com.” Id. Using state business regi stration records, ATF linked the TCGTR business and arfakit.com to Defendant.
A TCGTR 1 is a small piece of metal that , when properly bent according t o Defendant’s instructions, f its inside an AR-15. [T]he “TCGTR” (trigger control group travel reducer) was a cust ommade metal device, approximate ly 2.4 inches in length, and approximately 1/2 inch at its m ajor width. As sold by [Defendant], the . . . device required one[ ]fina l bend for the device to be come operational as a machinegun. T he purchaser could . . . go backto [Defendant’s] website . . . and obt ain the instructions on how to complete the device. T he device is premarked at the bend locat ion with a stencil. . . . [Defendant a lso] gave his customers very detailed written instructions, including p hotos, for making the final be nd to their [TCGTRs]. [Defendant] also s old a “raw materials” variat ion of his kit, which was the same kit, but a flat piece of metal that required a total of four bends. App. Vol. II at 18. As part of it s investigation, ATF ordered multiple TCGTRs from Defendant and, after following De fendant’s instructions for ben ding and installing a TCGTR, tested the effect on an AR-15.

1 The TCGTR is also referred to as a “kit” or “device.”4 “The AR-15 is the civilian vers ion of the military’s M-16 rifle , and is, unless modified, a semiautomatic weapon.” Staples v. United States , 511 U.S. 600, 603 (1994). A semiautomatic “weapon . . . fire s only one shot with each pul l of the trigger.” Id. at 602 n.1. Conversely, an “automa tic” or “fully automatic” “weap on . . . fires repeatedly with a single pull of the tri gger. That is, once its trigger i s depressed, the weapon will automatically continue to fire until its trigger is released or the ammunition is exhausted. Such weapons are ‘machineguns’ w ithin the meaning of the [Natio nal Firearms] Act[, 26 U.S.C. §§ 5801–5872].” Id. Based on its testing, ATF conc luded that the TCGTR was a machinegun because it caused an AR-15 to “fire[] automatically more than one round without manually reloading it with a single function of the tri gger.” App. Vol. III at 468. In December 2016, Defendant was indicted for violating 26 U.S.C . § 5861(a) by “knowingly engag[ing] in the bu siness of manufacturing and deal ing in firearms, to wit: machinegun conversion devices fo r AR-15 style rifl es, without h aving paid the special occupational tax . . . and without h aving registered” with the federal government. App. Vol. I at 14. Defendant was also indicted for violating 18 U.S .C. § 922(o) by “knowingly possess[ing] and transferr[ing] m achineguns, to wit: machinegun conversion devices for AR-15 style rifles.” Id. Defendant represented himself at the jury trial with the assistance of standby counsel. A t the hearing held pursuant to Faretta v. California , 422 U.S. 806 (1975), Defendant acknowl edged that he w ould “be requi red to comply with all of the court rules and the Rules of Procedure and the Rules of Evidence and all of the rules and procedures that pertai n in a jury trial.” App. Vol. III at 4.5 At trial, the government called AT F Special Agent Michael Powel l to testify as an expert about why the TCGTR was a machinegun. He explained that an AR-15 contains a part called a disconnector that “prevent[s] the firearm from fi ring a second shot without [the operator] releasing the tr igger and then pulling it a seco nd time.” Id. at 486. Powell then explained that the TCGTR “overrides or negates the functio n of the disconnector while the trigger is pulled a nd the weapon is” firing. Id. He testified that, by disabling the disconnector, the TCGTR allows an AR-15 to “fire[] automati cally.” Id. at 468. In Agent Powell’s expert opinion, thi s meant that the TCGTR met “the statutory definition of a machinegun” in 2 6 U.S.C. § 5845(b). Id. at 469. When it was Defendant’s turn to p resent his case, he testified about his design for the TCGTR. Defendant acknowledge d that he designed the TCGTR “ for an increased rate of fire,” but maintained th at he “wanted to do it legally. ” Id. at 553. Defendant then testified that he designed the T CGTR “to contact the trigger gr oup assembly,” though ostensibly without disabling the disconnector. Id. at 554. When Defe ndant began to testify about what part of the “ trigger group assembly” the TCG TR was designed to contact, he asked the court whet her he could “show the jury an animation that is on YouTube showing how that trigger group works.” Id. at 555. Defendant also offered to draw a diagram of the tri gger group on a whiteboard. The government objected, arguing that the testimony “presents b oth technical and specialized knowledge and under Rule 702 it would be required t o come in through a qualified expert witness.” Id. The district court sustained the government’s objection because it found that Defendant ’s testimony concerned “technica l and specialized6 knowledge,” id. at 561, but had not been disclo sed as required by Federal Rule of Criminal Procedure 16. The court acknowledged that Defendant h ad “the right to present a defense,” but noted that Defe ndant also had “an obligation to follow the rules, the Rules of Criminal Procedure.” Id. at 564; accord id. at 572. Defenda nt concluded his testimony and rested his case shortly thereafter. The jury returned a guilty verdi ct on both counts. Defendant w as sentenced to 33 months’ imprisonment, followe d by 36 months’ supervised rele ase. Defendant timely appealed his convictions. 2
II Defendant argues that the distric t court erred when it sustaine d the government’s objection to his testimony about how he intended the TCGTR to i nteract with the AR-15 trigger mechanism. The district court excluded this testimony after finding that it was expert testimony subject to Fed eral Rule of Evidence 702, but t hat Defendant had not timely disclosed his expert testi mony to the government, as required by Federal Rule of Criminal Procedure 16. Defendant further argues that the distr ict court’s limitation on his testimony violated his Fifth and Sixth Amendment right to prese nt a defense.

2 Defendant also moved for rele ase on bail pending appeal. He a rgued that this appeal raised a substantial ques tion of law likely to result in reversal, namely that “the district court violated [hi s] Fifth Amendment right to tes tify on his own behalf” about his intent when manufactu ring the TCGTR. Dkt. No. 106054 90 at 7. We denied the motion because, even though the district court “woul d not allow [Defendant to] . . . testi[fy ] that the trigger system in fact . . . was not a machinegun,” “the [district] court permitted [ Defendant] to testify about hi s intent.” Dkt. No. 10612197 at 2.7 A defendant’s right to present a defense is cabined by the Fede ral Rules of Evidence and Crim inal Procedure. The Fifth and Sixth Amendments g rant a defendant the “right to testify, present witnesses in h is own defense, and cross-examin e witnesses against him—often colle ctively referred to as the rig ht to present a defense.” United States v. Markey , 393 F.3d 1132, 1135 (10th Cir. 2004). But this right i s not absolute; a defendant must still “abide the rules of evide nce and procedure.” United States v. Dowlin , 408 F.3d 647, 659 (10th Cir. 2005) . . . . In light of the need to satisfy evidentiary requirements, [a defendant] bears a two-part bur den on [his] constitutional clai m. First, [he] must demonstrate that the district court abused its discretion in excluding the evidence. Dowlin , 408 F.3d at 659. Second, [he] must demonstrate that the excluded evidence “was o f such an exculpatory nature that its exclusion affected the tria l’s outcome.” Id. United States v. Tapaha , 891 F.3d 900, 905 (10th Ci r. 2018) (brackets and some citations omitted). When the government disclosed its expert witness prior to trial , it “request[ed] the disclosure of Defendant’s experts, ” “pursuant to Federal Rule o f Criminal Procedure 16(b)(1)(C).” App. Vol. I at 33. T his request triggered Defen dant’s obligation to “give to the government a written summ ary of any testimony that [he] intend[ed] to use under Rule[] 702.” 3 Fed. R. Crim. P. 16 (b)(1)(C). Defendant did not provide a s ummary of his testimony to the government. The refore, the district court onl y erred when excluding a

3 The parties do not discuss wheth er the government disclosed it s expert witnesses in response to “[D]efe ndant[’s] request[ for] disclos ure under subdivision (a)(1)(G)” of Rule 16. Fed. R. C rim. P. 16(b)(1)(C)(i). Becau se Defendant does not address this point, his only cha llenge to the applicability of Rule 16(b)(1)(C) turns on whether or not his testimony was expert testimony under Rule 70 2.8 portion of Defendant’s testimony if it was not expert testimony governed by Rule 702. 4
United States v. Adams , 271 F.3d 1236, 1243 (10th Ci r. 2001) (holding that exclusion of expert testimony because it was not disclosed to the government in a timely manner was an “evidentiary ruling . . . review [ed] for abuse of discretion ” even though the defendant argued that it denied him the right to present a defense). Defendant argues that Rule 702 d oes not apply because he would have offered “lay [testimony] as to the tec hnical aspects and functionalitie s of [his] device.” Aplt. Br. at 20. “A person may testify as a lay witness only if his opin ions or inferences do not require any specialized knowledge and could be reached by any o rdinary person.” United States v. Yeley-Davis , 632 F.3d 673, 684 (10th Cir. 2 011) (brackets omitted) (quotin g LifeWise Master Funding v. Telebank, 374 F.3d 917, 929 (1 0th Cir. 2004)). Lay testimony is “not based on scie ntific, technical, or other spec ialized knowledge within the scope of Rule 702.” Fed. R. Evi d. 701(c). “[T]he distinction between lay and expert witness testimony is that lay te stimony results from a process of reasoning familiar in everyday life, while expert tes timony results from a process of reasoning which can be mastered only by specialists in the field.” Fed. R. Evid. 701 advisory committee’s note to 2000 amendment (quotation marks omitted). “[P]rototypical exam ples of [lay testimony include] . . . the appearance of p ersons or things, identity, t he manner of conduct,

4 Defendant does not argue that the district court erred by excl uding a portion of Defendant’s testimony inste ad of imposing a lesser sanction or granting a continuance to allow the governme nt to prepare for Defendant’s testimony.9 competency of a perso n, degrees of light o r darkness, sound, si ze, weight, [and] distance . . . .” Id. (quotation marks and brackets omitted). Defendant wanted to testify that he designed the kit so that when a notched TCGTR is pushed down bya forward moving bolt carrier, it pushes down on the trigger bar causing the trigger to move back into its reset position. Because the operator is still trying to put pressure on the trigger, the op erator will pull the trigger almost as soon as it is back in the reset posi tion. The TCGTR in this configuration is c alled a forced reset trigger sy stem, otherwise known as a positive re set trigger system. It require s a separate trigger pull to fire each round. App. Vol. III at 570. Understandi ng Defendant’s testimony “req uire[d] . . . specialized knowledge” about how an AR-15 wo rks and how its component parts can be manipulated to increase the gun’s rate of fire. Yeley-Davis , 632 F.3d at 684 (quoting LifeWise Master Funding , 374 F.3d at 929). Moreover, the j ury would have needed to un derstand the significance of Defendant’s refere nce to a “forced reset trigge r system” and a “positive reset trigger system,” as well as how such systems differ from the analogous system in a machinegun. App. Vol. III at 570. T his is not knowledge “read ily accessible to any ordinary person.” Yeley-Davis , 632 F.3d at 684. Therefore, the district court did not abuse its discretion when finding that this portion of Defendan t’s testimony was expert testimony subject to Rule 702. Id. (holding that “testimony co ncerning how cell phone towers operate constituted expert testimony”). The fact that Defendant invented the TCGTR does not alter our c onclusion that he needed to comply with Rule 16’s disclosure requirement. None o f the cases cited by Defendant hold that a n inventor is exempt from Rule 702. Inste ad, they stand for the10 unremarkable proposition that an inventor may testify as a lay witness and, if properly qualified and disclosed, as an expert witness. See, e.g. , Verizon Servs. Corp. v. Cox Fibernet Va., Inc. , 602 F.3d 1325, 13 39–40 (Fed. Cir. 2010) (holding that a “district court did not abuse its discretion in limiting inventor testimony to factual testimony that did not require expert opinion” because the witnesses “had not previous ly provided expert reports or been qualified as . . . expert[s]”). Moreover, the district court’s limitation on Defendant’s testim ony did not prevent him from “directly address[ing a ] core issue[ in the case]—his intent.” Aplt. Br. at 24. After ruling on the scope of Defe ndant’s testimony, the distric t court allowed him to continue to present his case to the jury. Defe ndant concluded his testimony by telling the jury the following: What would I like you as the j ury to know about this kit? I guess, again, I would start that I am its creator and I am the manufacturer of it. My design for it was completely different than what the prosecution has alleged . My intent for it is complete ly different than what the prosecu tion has alleged. It was intend ed as an educational experience. I desi gned these kits to fire one r ound for each pull of the trigger, and I sen t each one of these kits out in a form that they couldn’t do anythi ng to an AR-15, maybe besides causing them to jam. I told peopl e not to complete the kit, ju st use the information, the education . Some people chose to complete their kit in a way that I did not int end, and a choice that each of them had a right to make. Not my choice. I will stand here and continue t o tell you point-blank, as the designer and manufacturer of this kit, that it is my absolute 100-percent belief that I did not make a machine gun, and that because of that I am no t guilty of the charges leveled against me by the government.11 App. Vol. III at 586–87. Therefore , Defendant was able to test ify that he did not intend to create a machinegun. T he district court’s ruling onl y prevented him from testifying about how the interac tion between a TCGTR and an AR- 15’s trigger mechanism alters an AR-15’s ra te of fire. Such testimony might have strengthened Defendant’s argument regarding intent, but that does not excuse Defendant from complying with Rule 16 before offering expert testimony. Tapaha , 891 F.3d at 905. Finally, Defendant argues that he “cannot be foreclosed from” “ elicit[ing] particularized and technical tes timony from lay witnesses”—name ly, himself—because “[t]he government made the strat egic decision to adduce testimo ny from lay witnesses as to the technical aspects and functionalities of” the TCGTR. Ap lt. Br. at 20–22. Even if it is true that the government im properly elicited expert testimon y from lay witnesses, Defendant does not raise that a lleged error as an issue on appe al. See Fed. R. App. P. 28(a)(5), (8). Defendant merely us es the alleged error as an e xcuse for him to disregard the Rules of Evidence and Criminal Procedure. But one party’s failure to comply with the Rules does not alter the oth er party’s obligation to follow the Rules. Otherwise, trials could rapidly devolve into free -for-alls. As Defendant acknowl edged when he asserted his right to represent himself at trial, “[t]he right of self-r epresentation is not a license . . . . to [disregard] relevant rule s of procedural and substan tive law.” United States v. McKinley , 58 F.3d 1475, 1483 (10th Cir. 1995) (quoting Faretta , 422 U.S. at 834 n.46).12 III Defendant also raises three unpr eserved issues, which we review for plain error. 5 To prevail under plain error review , Defendant must satisfy fou r requirements: First, there must be an error that has not been intentionally relinquished or abandoned. Secon d, the error must be plain—tha t is to say, clear or obvio us. Third, the error must have affected the defendant’s substantial rights, w hich in the ordinary case mean s he or she must show a reasonable probability that, but for the err or, the outcome of the proceeding would have been different. Once thes e three conditions have been met, [we] should exercise [our] disc retion to correct the forfeited error if the error seriously affects the fairness, integrity or public reputation of judicial proceedings. United States v. Giannukos , 908 F.3d 649, 654 (10t h Cir. 2018) (quoting MolinaMartinez v. United States , 136 S. Ct. 1338, 1343 (2016)).
A Defendant argues that the distric t court erred when instructing the jury on the elements of Count 1 of the Indictment, which alleged that Defen dant violated 26 U.S.C. § 5861(a) by unlawfully manufac turing machineguns. Specificall y, Defendant argues that the district court failed to instruct the jury on the requ isite mens rea. Section 5861(a) makes it “unlawful for any pers on . . . to engage in business a s a manufacturer . . . of . . . firearms without having paid t he special (occupational) tax . . . or having registered” with the federal government. A machin egun is a firearm. 26 U.S.C. § 5845(a).

5 In one footnote, Defendant argue s that “the aggregate effect o f the errors is not harmless” “under a cumulative error analysis.” Aplt. Br. a t 46 n. 15. Defendant’s brief discussion of t his issue is insufficient to g arner review on appeal. United States v. Cooper , 654 F.3d 1104, 1128 (10th Cir. 2011).13 Instruction Number 13 set out the elements of Count 1: 1. The defendant was e ngaged in business as a manufacturer of machineguns; 2. The defendant engaged in suc h business without having paid the special (occupational) tax or havi ng registered as required by federal law; and 3. The defendant kne w he was manufacturing machineguns. Supp. App. at 15; see also App. Vol. III at 632. “The term ‘ma chinegun’” was defined in Instruction Number 14 . Supp. App. at 16. Section 5861(a) does not incl ude a mens rea provision. See Staples , 511 U.S. at 605 (explaining that § 5861(d ) “is silent concerning the mens rea required for a violation”). In Staples , “the Supreme Court . . . held . . . that the government must prove a defendant knew the weapon he possessed had the characteristic s which made it a statutory firearm” to convict the defendant of violating § 5861 (d). United States v.
Mains , 33 F.3d 1222, 1229 (10th Cir. 1 994). We have not previously discussed whether the same mens rea requirement app lies to a prosecu tion under § 5861(a). The government agrees with Defendant that the mens rea requirement recognized in Staples should apply to a prosecution under § 5861(a) because “§ 5861(a ) and (d) are structurally identical.” Aple. Br. at 45; see also Aplt. Br. at 27 n.8 (asserting same structural argument). For the pur poses of this appeal, we assume that to be true. Defendant’s argument hinges on th e third element of InstructionNumber 13— “[t]he defendant knew he was manufacturing machineguns.” Supp. App. at 15. Defendant argues that this instruction “did not require the jur y to find that [Defendant]14 kn[e]w the specific physical tra its that make his device a mach inegun,” Aplt. Br. at 37, because “[i]t is not clear wha t word ‘knew’ . . . modifies—‘man ufacture’ or ‘machineguns,’” id. at 39. 6 In Defendant’s view, “[t]he l ogical and natural interpretatio n of the instruction is that [the ] jury need[ed] only find [Defen dant] knew he was manufacturing something.” Id. The government argues that “knew” modifies “manufacturing” and “machineguns.” 7 Aple. Br. at 46–49. Defendant is not entitled to relie f because any error in the ju ry instructions was not clear or obvious. “An error is pl ain if it is clear or obvious under current, well-settled law. In general, for an error to be contrary to well-settled l aw, either the Supreme Court or this court must have addressed the issue.” United States v. Justice , 679 F.3d 1251, 1256 (10th Cir. 2012) (quoting United States v. Thornburgh , 645 F.3d 1197, 1208 (10th Cir. 2011)). Defendant has not “ point[ed] to any authority fro m this court or the Supreme

6 Defendant argues that the instr uctions for Count 2, in which D efendant was charged with possessing or trans ferring machineg uns in violatio n of 18 U.S.C. § 922(o), better complies with Staples . With respect to Count 2, the jury was instructed that it n eeded to find that “[D]efendant knew the it em he possessed or transferred was a machinegun.” Supp. App. at 18. Even if ther e is a significant difference between the instructio ns for Counts 1 and 2, Defenda nt has not shown why that would affect the outcome o f his appeal. The jury convicte d Defendant on Count 2 and we now affirm its finding that Defendant knew the items h e possessed or transferred were machineguns. Because Defendant manufactured t he machineguns that he subsequently possessed a nd transferred, it would have b een inconsistent for the jury to find that Defendant d id not know the items he was m anufacturing were machineguns.

7 This argument is cons istent with the govern ment’s position at trial. In its closing argument, the governmen t told the jury that, to convict Defendant of count one, it needed “to consider . . . whether [Defendant] knew what he was manufacturing was a machine gun.” App. Vol. III at 642.15 Court to support []his proposition.” 8 Id. at 1257. To the contrary, we have held that a similarly worded instruction complies with Staples and adequately instructs a jury on the required mens rea for a § 5861 prosecution.
In Mains , a defendant was charged with v iolating 26 U.S .C. § 5861(d) an d the jury was instructed that it needed t o find that “the defendant . . . knowingly possessed a shotgun with a barrel length o f less than 18 inches or an overa ll length less than 26 inches.” 33 F.3d at 1229 (quota tion marks and emphasis omitted ). We held that this instruction, “in accord with Staples , . . . properly required the ju ry to find [the d]efendant knew the particular characteris tics which made his sawed-off sh otgun a statutory firearm.” Id. We reached that conclusion be cause “the jury was required to find that [the d]efendant (1) possessed a s awed-off shotgun with a barrel length of less than eighteen inches or an overall leng th less than twenty-six inche s, and (2) knew he possessed a sawed-off shotgun with a barrel length of less than eighteen inches or an overall length less than twenty-six inches.” Id. Therefore, in Mains , we understood

8 Defendant instead points to a p air of cases from other circuit s, only one of which concerns a prosecution under § 5861. See United States v. White , 863 F.3d 784, 786 (8th Cir. 2017) (en banc) (analyzing jury instruction for prosecution under § 5861(d)); United States v. Ahmad , 101 F.3d 386, 389 (5th Cir. 1996) (analyzing jury instruction for prosecution under the Clean Water Act). T he primary concern in White , 863 F.3d at 792, and Ahmad , 101 F.3d at 391, was tha t the mens rea element was contained in its own line of text, and therefore physically separate from the other relevant elements of the crime. That problem is not present he re. The instruction in this case—“[t]he defendant knew he was manufacturing machinegun s”—contains the mens rea element (“knew”) in the same line as the characteristi cs of the firearms (“machineguns”). Therefore, the cases cited by Defendant do no t establish that any instructional error wa s clear or obvious.16 “knowingly” to modify the entir e phrase that followed, not just the immediately following word. 9 A plurality of the Supr eme Court reached the same conclusion wi th respect to a similar instruction. In Rogers v. United States , a defendant was charged with violating 26 U.S.C. § 5861(d) and the distric t court instructed the jury tha t it needed to find that “the [d]efendant knowingly possessed a firearm, as defined” in a pre vious instruction to include a silencer. 522 U.S. 252, 257 (1998) (plurality opinio n) (quotation marks omitted). After the defendant w as convicted, the Supreme Court decided Staples

. Id. at 254 n.1. The plurality concluded that, “ [s]ince the term‘firearm’ had b een ‘defined above’ to include a silencer, that instruction required the jury to de termine that the defendant knew that the item he possessed was a silencer.” Id. at 257. The plu rality “assume[d] that the trial judge would have b een more explicit in explainin g the mens rea element of the[] offense[] if Staples had been decided prior to subm itting the case to the jury,” bu t was “satisfied that the instruc tions as given did inform the ju rors that they must find that the defendant knew that the sile ncer was in fact a silencer.” Id. at 258. The dissenting Justices disagreed, arguing that “[t]he word ‘knowingly’ in the instruction modifies the word which follows it, viz., ‘po ssessed,’ rather than the instr uction’s further reference to the statutory definition of ‘firearm.’” Id. at 260 (Kennedy, J., dissen ting). In closing, the

9 Defendant “does not concede that the Mains instruction suffices under Staples .” Aplt. Br. at 41 n.12. But t here is nothing for Defendant t o concede. In Mains , we held that the jury instr uction at issue complied with Staples . 33 F.3d at 1229.17 Rogers plurality cautioned that “[i]t w ould be wise for trial courts t o explain the Staples requirement more carefully tha n the instruction used in [ Rogers ] to foreclose any possibility that jurors might int erpret the instruction as [the ] . . . dissent [did].” Id. at 258 n.7 (plurality opinion). Because of our holding in Mains , any error in Instruction Number 13 was not clear or obvious. That being said, we echo the Rogers plurality’s comme nts about the specificity with which a district court should instruct a jury regarding a defendant’s knowledge of “the features of h is [firearm] that br[ing] it within the scope of the [NFA].” Staples , 511 U.S. at 619; see also Pattern Crim. Jury Instr. 10th Cir. § 2.91 (2011 ed. 2018) (setting out pattern jury in struction for a prosecution under § 5861(d)).
B. Defendant argues that the distr ict court erred in allowing the government to ask witnesses to “repeat[] out-of-cou rt statements that ATF believe d [Defendant’s] devices were illegal.” Aplt. Br. at 31 . The government called as witn esses seven people who purchased a TCGTR from Defendant . ATF agents visited many of t hese customers during their investigation and to ld the customers that the TCGT R was illegal. The government elicited testimony fro m four of the customers about the ATF agents’ prior statements. Because the governmen t does not dispute that these statements were inadmissible hearsay, we assume t hat the first two requirements of the plain error test are satisfied.18 Instead, the government argues th at Defendant cannot satisfy hi s burden under the plain error test because the erro r did not affect his substanti al rights. 10 We have repeatedly held that improperl y admitted hearsay testimony is h armless when cumulative of properly admitted testimony. United States v. Garcia , 793 F.3d 1194, 1214–15 (10th Cir. 2015); United States v. Brooks , 736 F.3d 921, 936 (10th Cir. 2013); see also United
States v. Martinez , 166 F.3d 1222, at *7 (10th Ci r. 1999) (unpublished table deci sion) (collecting cases). Here, the g overnment called ATF Agent Mich ael Powell to testify at length about his expert opinion that the TCGTR was illegal because it was a machinegun. Defendant had the opportunity to cross-examine Powell and explo re how and why he concluded that the TCGTR was illegal. Therefore, because the f our brief and conclusory hearsay statements admitted thr ough the customer-witnesses were cumulative of Agent Powell’s testimony, Defendant has not shown that any error affe cted his substantial rights.
C. Defendant argues that the distric t court erred by allowing Agen t Powell, an expert witness for the government, to te stify: The TCGTR “meets the st atutory definition of a machinegun that is found in 26 United States Code Section 5845( b). That definition includes any part designed and in tended solely and exclusively for use in converting a

10 The government also argues that Defendant waived any objection to the challenged statements because he cross-examined the witnesses a bout the ATF’s warnings that the TCGTR was ille gal. We need not reach the Gov ernment’s waiver argument because we conclude tha t any error in admitting the ch allenged statements did not affect Defendant’s substantial rights.19 weapon into a machinegun. This part meets that definition.” A plt. Br. at 34 (quoting App. Vol. III at 469). Defendant argues that this testimony wa s improper because “experts may not opine on ultimat e issues of fact without unpac king the technical or specialized criteria upon wh ich the opinion is based.” 11 Id. at 33. “An opinion is not objectionable just because it embraces an ul timate issue.” Fed. R. Evid. 704(a). Moreover, “[w]i tnesses are permitted to testi fy about how the law applies to a certain set of facts , so long as they provide adeq uate explanations for their conclusions.” United States v. Richter , 796 F.3d 1173, 1196 (1 0th Cir. 2015). Therefore, as long as Agent Powell explaine d the basis for his opinion, it was not improper for him to testify that the TCGTR was a machinegun. United States v. Buchanan , 787 F.2d 477, 483–84 (10th Cir. 1986) (holding th at an expert witness may tes tify that “a particular device . . . must be registered” as a firearm with ATF).

11 When later discussing the prej udice he allegedly suffered from Agent Powell’s testimony, Defendant als o argues that Agent Powell imp roperly “testified as to [Defendant’s] intent.” Aplt . Br. at 45. “In a criminal cas e, an expert witness must not state an opinion about wheth er the defendant did or did not have a mental state or condition that constitutes an elem ent of the crime charged . . . .” Fed. R. Evid. 704(b). But Agent Powell did not testify that Defendant knew t he TCGTR was a machinegun when he manufactured t he device. Agent Powell testi fied about his examination and testing of the TCGTR, including his opinions co ncerning the effect that a TCGTR has on an AR-15’s rate of fire. Based on his anal ysis, Agent Powell offered his expert opinion that a TCGTR’s only purpose is to co nvert an AR-15 into a machinegun. Consistent with Rule 704(b), the jury was then abl e to evaluate Agent Powell’s testimony when “conclud[ ing] or infer[ring] th[at] [D] efendant had the requisite mental state” to be c onvicted of violating § 5861(a) and § 922(o). United States v. Archuleta , 737 F.3d 1287, 1297 ( 10th Cir. 2013) (quoting United States v. Goodman , 633 F.3d 963, 970 (10th Cir. 2011)).20 Agent Powell thorough ly explained the factual basis for his opi nion that the TCGTR is a machinegun. On direct examination, the government a sked Agent Powell “to explain how [the TCGTR] conv erts [a] firearm into a machine gun.” App. Vol. III at 470. Powell then conducted a d emonstration for the jury about how the TCGTR worked inside an actual AR-15, explaini ng and showing that the TCGTR w ould cause multiple bullets to be fired without “releasing the trigger.” Id. at 472. Defendant followed up on this testimony during cross-exam ination. In a preface to one o f his questions, Defendant stated that he understood Powell’ s opinion to be that the devic e “pushes down the disconnector which releases the hammer and fires the weapon.” Id. at 486. Powell confirmed that Defendant’s under standing was correct, stating: “[T]his device overrides the function of the disconnecto r, which is supposed to prevent you from getting a second shot with a single function of t he trigger, but this device act ually overrides or negates the function of the disconnector wh ile the trigger is pulled and th e weapon is” firing. Id. Powell also testified about why h e considered the TCGTR to be amachinegun when it was shipped to custome rs without any of the required be nds. He explained that the unbent TCGTR was s till a machinegun “[b ]ecause of the relat ive ease that it took to do those bending operations.” Id. at 474. When a custome r purchased an unbent TCGTR, they could obtain instr uctions for how to make the requi red bends and install the TCGTR into an AR-15. It took Powe ll “approximately five minute s to do the four bending operations” needed to complete the device. Id. at 477. As Powell also explained, the ease of use did not appear accidental. The “dev ice has to be the perfect length in order to function proper ly in [an AR-15],” “has to be cut to the perfect width,”21 and needs “clearance cuts” or “co ntouring” “to fit inside of th [e AR-15] without binding . . . [and to otherwise] work properly.” Id. at 474. Because Agent Powell adequately explained the basis for his opini on that the TCGTR is a machine gun, as defined in § 5845(b), the distric t court did not err in allowing his testi mony. IV We AFFIRM.