No Plain Error from “Elements Met” Nexus-Expert Testimony Under Rules 704(b)/403; § 922(g)(1) As-Applied Challenges Foreclosed by Vincent v. Bondi

I. Introduction

In United States v. McBee (10th Cir. July 14, 2026), Mario Terrell McBee appealed his jury conviction for being a felon in possession of a firearm and ammunition under 18 U.S.C. § 922(g)(1). The case arose after hospital staff discovered a loaded handgun in McBee’s waistband and ammunition on his person when he arrived unconscious at the Claremore Indian Hospital in Oklahoma. Investigation confirmed prior felony convictions.

On appeal, McBee raised three issues:

• Rule 704(b): Whether the admission of ATF Agent Brett Williams’s expert testimony—particularly his statements that “all the elements of the crime had been met”—impermissibly opined on McBee’s mens rea.
• Rule 403: Whether that same testimony unfairly prejudiced McBee by effectively instructing the jury on the law or implying guilt.
• Second Amendment (as-applied): Whether § 922(g)(1) is unconstitutional as applied to McBee.

Because McBee did not contemporaneously object to the testimony at trial, the evidentiary issues were reviewed only for plain error. The constitutional claim was preserved and reviewed de novo, but was conceded to be foreclosed by circuit precedent.

II. Summary of the Opinion

The Tenth Circuit affirmed. It held:

• The Rule 704(b) challenge failed under plain-error review because any error was not plain; it was not “clear or obvious under current, well-settled law” that the agent’s “elements met” remarks were an expert opinion on mens rea.
• The Rule 403 challenge likewise failed for lack of a plain and obvious error; the court found it unclear that the testimony ventured beyond nexus expertise or created unfair prejudice substantially outweighing probative value.
• The as-applied Second Amendment challenge to § 922(g)(1) was foreclosed by Vincent v. Bondi, which held § 922(g)(1) may constitutionally be applied to nonviolent offenders.

III. Analysis

A. Precedents Cited

1. Plain-error framework and “plainness” requirement

• United States v. Garza, 566 F.3d 1194 (10th Cir. 2009): Used to establish that unpreserved evidentiary objections are reviewed for plain error.
• United States v. Archuleta, 737 F.3d 1287 (10th Cir. 2013): Applied as authority for plain-error review specifically in the Rule 704(b) context.
• United States v. Olano, 507 U.S. 725 (1993): Provides the canonical four-part plain-error test (error; plain; affects substantial rights; discretionary correction for integrity/fairness).
• United States v. B.N.M., 107 F.4th 1152 (10th Cir. 2024): Quoted for the discretionary fourth prong—correction when an error “seriously affects the fairness, integrity, or public reputation of judicial proceedings.”
• United States v. Faulkner, 950 F.3d 670 (10th Cir. 2019) and United States v. Cantu, 964 F.3d 924 (10th Cir. 2020): Central to the outcome; they frame “plain” as “clear or obvious under current, well-settled law.” The panel used this to reject McBee’s attempt to characterize the evidentiary admission as plainly erroneous.
• United States v. Doe, 154 F.4th 777 (10th Cir. 2025): Cited to justify declining analysis of other plain-error prongs once “plainness” fails.

2. Rule 704(b) (expert opinions on mens rea)

• United States v. Maryboy, 138 F.4th 1274 (10th Cir. 2025): The key Rule 704(b) precedent invoked by McBee. The panel extracted Maryboy’s test: expert testimony violates Rule 704(b) when it asserts a final conclusion that the defendant had the requisite mens rea, and the “critical question” is whether it permits the factfinder to make an additional inference regarding mental state. The court distinguished Maryboy on its facts and held it did not make any error here “clear or obvious.”
• Diaz v. United States, 602 U.S. 526 (2024): Used to underscore that no Rule 704(b) error occurs if the expert does not “necessarily describe” the defendant’s mental state. This case supported the court’s view that Williams’s nexus-focused testimony did not plainly address McBee’s mens rea.
• United States v. Finnesy, 953 F.3d 675 (10th Cir. 2020): Cited for the proposition that sometimes the “plain terms” of a rule/statute can establish plain error. The panel acknowledged the concept but found the rule’s text, standing alone, did not make the alleged error obvious here.

3. Elements of § 922(g)(1) and scienter

• United States v. Trujillo, 960 F.3d 1196 (10th Cir. 2020): Provided the statement of the elements for § 922(g)(1) as applied by the panel (felony status; knowing possession; knowledge of status; interstate commerce).
• Rehaif v. United States, 588 U.S. 225 (2019): Referenced to situate the scienter requirement, especially knowledge of status, though McBee stipulated to that element.

4. Rule 403 and expert-scope concerns

• The court discussed Rule 403 directly (probative value vs. unfair prejudice/confusion) and referenced Rules 702 and 704 because McBee argued the expert strayed into legal conclusions. No single additional precedent controlled; rather, the panel’s Rule 403 holding largely followed from its Rule 704(b) “non-obviousness” analysis and the absence of a clear showing that the jury would treat “elements met” as an authoritative directive on guilt.

5. Second Amendment and § 922(g)(1)

• United States v. Harjo, 122 F.4th 1240 (10th Cir. 2024): Cited for de novo review of preserved constitutional challenges.
• Vincent v. Bondi, 127 F.4th 1263 (10th Cir. 2025): Dispositive; the panel treated it as binding precedent foreclosing McBee’s as-applied challenge, stating that § 922(g)(1) may constitutionally be applied to nonviolent offenders.

B. Legal Reasoning

1. Why the Rule 704(b) claim failed on “plainness”

The panel’s reasoning is best understood as a two-step uncertainty analysis, either of which prevents a finding that any error was “clear or obvious”:

• Unclear that Williams opined on mens rea at all: Although Williams said he believed “all the elements of the crime had been met,” the testimony was embedded in a narrative about his role as a “nexus specialist” (interstate-commerce analysis) and his decision not to do additional investigation. The court emphasized the testimony did not reference McBee’s mental state or suggest mental state was relevant to nexus analysis. The court also looked to the government’s closing argument, noting the government still treated knowing possession as disputed rather than relying on Williams to establish mens rea (a consideration drawn from Maryboy).
• Unclear that the jury would treat “elements met” as a final mens rea conclusion: The panel highlighted a “mismatch” between Williams’s colloquial phrase “elements of the crime” and the jury instructions, which did not use that same framing in instructing the jurors on “knowingly.” The court also noted Instruction 9 described Williams’s opinions as concerning interstate commerce, which further reduced the likelihood the jury treated “elements met” as an expert verdict on guilt.

Taken together, these ambiguities meant McBee could not satisfy the second prong of plain error—plainness—even if a colorable Rule 704(b) issue existed.

2. Why the Rule 403 claim failed on “plainness”

McBee argued that an admitted expert’s assertion that “all the elements had been met” would cause jurors to defer and thus unfairly prejudice him. The panel rejected this, again on “plainness,” because:

• It was not clearly established that Williams was offering legal instruction or mens rea conclusions rather than explaining investigative sufficiency for grand-jury referral.
• Absent a clear link to mental state (and given the limiting description of Williams’s testimony as commerce-focused), McBee did not “clearly demonstrate” unfair prejudice, much less prejudice that substantially outweighed probative value.

3. Why the § 922(g)(1) as-applied challenge failed

The panel treated Vincent v. Bondi as controlling circuit law and stated it “compels” affirmance because it holds § 922(g)(1) can be constitutionally applied to nonviolent offenders. McBee preserved the issue but conceded the precedent foreclosed relief.

C. Impact

1. Practical impact on trial objections to expert testimony

The opinion reinforces a recurring appellate lesson: without a contemporaneous objection, a defendant faces a steep uphill climb. Even potentially problematic expert phrasing (e.g., “elements of the crime had been met”) may not yield relief unless the error is clearly prohibited by settled authority and unmistakably tied to an element reserved to the jury—especially mens rea.

2. Guidance on Rule 704(b) arguments after Maryboy

While Maryboy prohibits experts from supplying a “final conclusion” on mens rea, McBee indicates that generalized, process-oriented remarks about “elements” may evade plain-error correction where:

• the testimony’s subject matter is largely unrelated to mens rea (here, nexus/interstate commerce),
• the jury instructions cabin the expert’s role, and
• the government does not argue the expert proved mens rea in closing.

This does not mean such testimony is always admissible; rather, it shows appellate courts may refuse to label it plainly inadmissible without a timely objection and a clear doctrinal match.

3. Continued foreclosure of as-applied § 922(g)(1) claims in the Tenth Circuit

Unless and until Vincent v. Bondi is revisited by the en banc court or superseded by the Supreme Court, Tenth Circuit panels will treat as-applied Second Amendment challenges by nonviolent felons to § 922(g)(1) as foreclosed. The opinion signals the court’s willingness to resolve such claims succinctly on stare decisis grounds.

IV. Complex Concepts Simplified

• Plain-error review: A demanding appellate standard applied when the party did not object at trial. The appellant must show an error that is not just wrong but obviously wrong under settled law, and that it likely affected the outcome.
• Rule 704(b): In criminal cases, experts cannot opine on whether the defendant had the mental state that is an element of the crime (e.g., that the defendant “knowingly” possessed something). The jury decides mental state.
• Rule 403: Even relevant evidence can be excluded if it is likely to cause unfair prejudice, confuse issues, or mislead the jury, and that danger substantially outweighs its value.
• Nexus / interstate-commerce element (in § 922(g)(1)): The government must show the firearm or ammunition moved in or affected interstate commerce (often by proving it was manufactured out of state). A “nexus specialist” commonly testifies to that movement.
• As-applied constitutional challenge: An argument that a law may be valid in general but is unconstitutional when applied to this particular person and circumstances.

V. Conclusion

United States v. McBee affirms a § 922(g)(1) conviction while offering a practical, procedure-driven lesson: without timely objections, appellate courts will rarely grant relief from arguable expert overreach unless the violation is unmistakable under settled authority. The decision also underscores that, in the Tenth Circuit, as-applied Second Amendment challenges to § 922(g)(1) by nonviolent offenders remain foreclosed by Vincent v. Bondi.