Calling the question of whether or not bump stocks is a machine gun part a “close one on which reasonable jurists have disagreed” because of the ambiguity of 18 U.S.C. § 922(o)(1), the court nevertheless concluded that “[b]ecause the relevant statutory scheme does not clearly and unambiguously prohibit bump stocks, we are bound to construe the statute in Hardin’s favor.”

This brings us to the rule of lenity, under which “penal statutes are to be construed strictly.” Therefore, when Chevron deference is not warranted and standard principles of statutory interpretation “fail to establish that the Government’s position is unambiguously correct[,] we apply the rule of lenity and resolve the ambiguity in [the criminal defendant’s] favor.” “In sum, it is not enough to conclude that a criminal statute should cover a particular act. The statute must clearly and unambiguously cover the act.”

In this case, the differing opinions among “reasonable jurists” helped resolve the case in Scott Hardin’s favor. As Judge Ronald Gilman wrote in the majority opinion, the “viability of competing interpretations is exemplified not only by the myriad and conflicting judicial opinions on this issue, but also by the ATF’s own flip-flop in its position.” A concurring opinion by Judge John K. Bush went even further; agreeing that the lower court’s verdict needs to be reversed but taking issue with the ATF’s “flip-flop” on the legality of bump stocks.

The ATF had long maintained that bump stocks were not machine gun parts because they did not cause a gun to discharge more than one round with a single function of the trigger, but after the Las Vegas shootings in October of 2017 and “high profile statements from President Trump and others” the agency did an abrupt about-face in declaring that bump stocks can and should be treated like machine gun parts; in part by reading the phrase “single function of the trigger” to mean “single pull of the trigger.” As Judge Bush notes, [t]here were no changes in the relevant facts or law that led to the ATF making a 180-degree change of statutory interpretation to ban what once was legal. There was only a profound change in political pressure.”

Even if the ATF had adopted its current view from the get-go, that interpretation fits poorly with the statutory text. The ATF substitutes “pull” for “function” to argue that there is a single “pull” from the shooter’s perspective. But the statutory definition defines “function” not with reference to the shooter but to the firearm, given the use of the word “trigger,” which is a mechanical feature. From the firearm’s mechanical perspective, the trigger must fully reset and be “pulled” every single time another shot is fired, so substitution of the ATF’s new word, “pull” for “function,” does not make a bump-stock rifle a machinegun. Even with the bump stock, the trigger of the rifle still must be pulled—that is, the trigger finger must move against the trigger while the shooter maintains forward pressure on the weapon’s barrel-shroud or fore-grip—for each shot the weapon fires.

To be sure, the bump stock allows for multiple shots to occur more rapidly, but that consequence does not change the dispositive fact that each pull of the trigger fires only one shot. Because a single function of the trigger using a bump stock cannot fire more than one bullet, a bump-stock rifle is not a machinegun.

Bush gets it exactly right, and is far less amenable to the supposedly reasonable disagreements between the appellate courts cited by Gilman in the majority opinion. In his view there’s not much ambiguity in the federal statute, and it’s much more clear that the ATF was acting beyond the scope of its authority when enacting the ban. Both Gilman and Bush point out that its up to Congress to write the laws, not an administrative agency, but it sounds like Bush would have sided in favor with Hardin even if the rule of lenity didn’t apply.

The Biden administration is likely to appeal today’s decision to an en banc panel of Sixth Circuit judges, though with the Fifth Circuit already issuing an en banc decision rejecting the bump stock ban I suppose Merrick Garland could appeal directly to the Supreme Court instead. Given the current makeup of the Court, however, Garland will probably try to keep this case away from the justices for as long as possible in the hopes of keeping the bump stock ban alive in at least some parts of the country until the makeup of the Court is more amenable to gun control arguments and government overreach.