In 2017, a single gunman opened fire on a country music festival in Las Vegas from the window of his hotel room overlooking that festival. The carnage was extraordinary. Sixty people died, and hundreds more were wounded.

One reason this shooting was so deadly is that the shooter used a device known as a “bump stock,” which effectively allows widely available semi-automatic rifles to mimic automatic weapons (the term “semi-automatic” refers to guns that expel one bullet for each pull of the trigger). Thanks to this device, the Las Vegas shooter was able to fire about nine shots every second.

In response to this shooting — the deadliest mass shooting in recent US history — and others, even the Trump administration concluded that bump stocks are intolerable. Federal law makes it a crime to possess a “machinegun,” and a Justice Department rule that took effect in 2019 clarifies that a weapon that is modified by a bump stock to allow automatic fire qualifies as a machine gun.

Now, however, that rule is in grave danger. On Friday, the United States Court of Appeals for the Fifth Circuit handed down a decision striking down this Trump era rule. Given the Fifth Circuit’s political leanings — the Court routinely hands down legally dubious decisions implementing far-right policy goals — this outcome is not surprising. But, while much of the Fifth Circuit’s reasoning is difficult to defend, one of its arguments against the bump stock ban is plausible.

As the Fifth Circuit notes in its Cargill v. Garland opinion, moreover, other judges are starkly divided over whether federal law permits the Justice Department to ban bump stocks. Ten years ago, this would have been a slam dunk case in favor of the ban. While the law governing machine guns can be read in two different ways, the Court’s decision in Chevron v. Natural Resources Defense Council (1984) ordinarily requires judges to defer to a federal agency’s interpretation of a statute when that statute is ambiguous.

But decisions like Chevron — indeed, virtually all decisions calling on the justices to defer to anyone other than themselves — are very much out-of-favor on this Supreme Court. And, while Chevron technically has not been overruled, the Court’s six Republican appointees have largely abandoned it. (One member of the Court, Justice Neil Gorsuch, has argued that Chevron should not apply specifically to criminal statutes, but his opinion making this argument was joined by no other justice.)

And that means that the legality of the bump stock ban is genuinely uncertain, and that it will almost certainly be decided by a Supreme Court that is hostile both to gun laws and to regulatory actions by federal agencies like DOJ.

The uncertain legality of the bump stock ban, briefly explained

Most automatic weapons use an internal mechanism to repeatedly fire the weapon for as long as the shooter holds down the trigger. These weapons are clearly illegal under the federal ban on machine guns.

Bump stocks, by contrast, are external devices that use the gun’s own recoil to repeatedly pull the trigger, enabling a semi-automatic weapon to fire nearly as fast as a traditional automatic weapon. Essentially, they cause the gun’s trigger to buck against the shooter’s finger as the gun’s recoil causes it to jerk back and forth, repeatedly “bumping” the trigger and causing the gun to fire over and over again.

This particular mechanism matters because the federal machine gun ban plausibly can be read to exempt devices like it, that repeatedly pull a gun’s trigger. That law prohibits weapons that “automatically” fire more than one shot “by a single function of the trigger.”

Judges across the country have split on how to read this statute. A majority of the Fifth Circuit’s judges, the overwhelming majority of whom were appointed by Republican presidents, claim in Cargill that bump stocks successfully evade the federal machine gun ban. Though these judges concede that bump stocks allow semi-automatic weapons to be fired at an accelerated rate, they argue that “the fact remains that only one bullet is fired each time the shooter pulls the trigger.”

That’s certainly one plausible reading of the federal law, but, as multiple other courts have explained, the law can also be read a different way. As the DC Circuit explained in Guedes v. ATF (2019), the law’s reference to “a single function of the trigger” could also be read to mean “a single pull of the trigger from the perspective of the shooter.” Under this reading of the statute, bump stocks are illegal machine guns because “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.”

The Fifth Circuit, for its part, spends much of its opinion arguing that the statute is unambiguous, and can only be read in the way that most of its judges prefer. But that argument is untenable. The mere fact that so many federal judges do not share this reading of the statute — and the fact that the Trump administration believed that the alternative reading of the statute was, at least, reasonable — should have given the Fifth Circuit pause before it claimed to have discovered the only possible way to read this ambiguous law.

Under Chevron, moreover, when a federal law is ambiguous, courts typically should defer to a federal agency’s reading of that law. So, in this case, the Fifth Circuit should have deferred to the Justice Department’s conclusion that the ambiguously drafted machine gun ban does extend to bump stocks.

But, while the Court has not yet explicitly overruled Chevron, the Supreme Court’s most recent decisions governing federal agencies’ interpretations of federal law are inconsistent with Chevron and seem to reject the proposition that courts should defer to federal agencies — at least over matters the Court deems of “vast ‘economic and political significance.’” Some members of the Court have gone even further, calling for Chevron to be reconsidered.

One problem with this approach, however, is that the machine gun ban is genuinely ambiguous. So, if courts do not defer to the Justice Department’s interpretation of the law, it is unclear how, exactly, they are supposed to read the law. And that means that, without Chevron, there is considerable uncertainty about whether bump stocks are legal or not.

The strongest argument against the bump stock ban

Though most of the Fifth Circuit’s opinion rests on its poorly reasoned conclusion that the machine gun statute is unambiguous, the court does devote a few pages to a more defensible argument — that the federal statute should be read to exclude bump stocks because, as the Supreme Court held in Rewis v. United States (1971), “ambiguity concerning the ambit of criminal statutes should be resolved in favor of lenity.”

That is, when a criminal law can be read in multiple ways, some of which would render a criminal defendant’s actions illegal and some of which would not, the tie should go to the defendant. As a general rule, people should not be fined or sent to prison over an ambiguous law that does not adequately convey to them what sort of conduct is illegal.

The federal ban on machine guns carries a criminal penalty of up to 10 years behind bars. And, though the Cargill case does not involve a criminal prosecution — the plaintiff is a gun owner who previously owned bump stocks but gave them up after DOJ announced the ban — the Fifth Circuit’s decision would prohibit the government from prosecuting anyone caught with a bump stock.

But as Judge Stephen Higginson, an Obama appointee, writes in his Cargill dissent, there are several reasons to doubt whether the rule of lenity should apply to the bump stock ban. For one thing, as the Supreme Court said in Barber v. Thomas (2010), “the rule of lenity only applies if, after considering text, structure, history, and purpose, there remains a ‘grievous ambiguity or uncertainty in the statute,’ such that the Court must simply ‘guess as to what Congress intended.’”

Perhaps it would be better if the courts were to read the rule of lenity expansively to prohibit criminal statutes from being enforced if they are the least bit ambiguous, but this is certainly not the approach that the Fifth Circuit has taken in its previous decisions. In United States v. Palomares (2022), for example, the Fifth Circuit labeled a federal law “perplexing” before construing it against a criminal defendant.

It’s conceivable, in other words, that the deeply ideological Fifth Circuit is operating in bad faith, applying a permissive rule to gun owners that it would not apply to other criminal defendants. This certainly wouldn’t be the first time that Fifth Circuit judges bent the rules to achieve their preferred outcome.

But, despite the problems with the Fifth Circuit’s lenity analysis, this approach does have one clear virtue. Without Chevron, courts need to rely on something to determine how an ambiguous statute must be read, and an expansive rule of lenity, at the very least, enables courts to make this determination in criminal cases.

If that rule is to be taken seriously, however, it would need to apply to all criminal defendants. And not just to defendants charged with violating gun laws — or other laws that Republicans typically do not like.