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The Supreme Court will hear a case that could effectively legalize automatic weapons

The justices will decide if bump stocks, devices that effectively convert a semiautomatic gun into an automatic weapon, are legal.

A person wearing earmuffs and safety goggles holds a gun.
Spent shells fly out of an AK-47 fitted with a bump stock. The bump stock is a device that allows a semiautomatic to fire at a rapid rate much like a fully automatic gun.
George Frey/Getty Images
Ian Millhiser is a senior correspondent at Vox, where he focuses on the Supreme Court, the Constitution, and the decline of liberal democracy in the United States. He received a JD from Duke University and is the author of two books on the Supreme Court.

The Supreme Court announced on Friday that it will hear Garland v. Cargill, a case that could legalize a device that allows an ordinary (and legal) semiautomatic firearm to mimic a fully automatic machine gun that can spew multiple bullets every second.

Cargill involves bump stocks, devices that use a gun’s recoil to repeatedly pull its trigger, allowing the gun to fire as many as 90 bullets in just 10 seconds. Bump stocks cause a gun’s trigger to buck against the shooter’s finger while the gun’s recoil makes it jerk back and forth, “bumping” the trigger and causing it to fire again and again.

In 2017, a gunman opened fire on a country music festival in Las Vegas, killing 60 people and wounding hundreds more. One reason this shooting was so deadly is that the shooter used a bump stock.

In response, the Trump administration concluded that these devices should not be legal, and it issued a regulation in 2018 that determined that bump stocks violate a 1986 federal law making it a crime to own a “machinegun.” But there’s a problem: The 1986 law is ambiguous, and federal courts are divided on whether it should be read to allow this ban on bump stocks.

Had this case arisen before former President Donald Trump remade the Supreme Court in the Federalist Society’s image, it would be a very easy one. 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, and so Chevron would require judges to accept the Justice Department’s conclusion that bump stocks count as “machineguns.”

But the Trumpified Supreme Court plans to hear a case this term asking it to overrule Chevron. And the Court has already made clear, in cases applying its so-called “major questions doctrine,” that the judiciary has the power to veto regulations even when a federal statute unambiguously authorizes that regulation.

Which means that the fate of the bump stock ban is now quite uncertain.

So what does federal law actually say about bump stocks?

The 1986 law prohibits weapons that “automatically” fire more than one shot “by a single function of the trigger.” This wording matters because bump stocks cause the trigger of a semiautomatic weapon to repeatedly buck against a shooter’s finger — and it is unclear if the law is broadly worded enough to ban this mechanism.

Some courts, including the left-leaning United States Court of Appeals for the District of Columbia Circuit, have ruled that the bump stock ban is lawful. As that court argued in Guedes v. ATF (2019), the 1986 law’s reference to “a single function of the trigger” can be read to mean “a single pull of the trigger from the perspective of the shooter.” Thus, 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.”

Meanwhile, the right-wing Fifth Circuit reached the opposite conclusion. Though the Fifth Circuit conceded that bump stocks allow semiautomatic weapons to be fired at an accelerated rate, it claimed that “the fact remains that only one bullet is fired each time the shooter pulls the trigger.”

And then there’s the Sixth Circuit’s decision, which was authored by centrist Clinton Judge Ronald Gilman. Gilman concluded that the 1986 law is, indeed, ambiguous. Then he struck down the bump stock ban by applying something known as the “rule of lenity,” which sometimes establishes that when a criminal law is ambiguous, it should be construed in favor of the defendant.

One problem with the Sixth Circuit’s approach, however, is that the Supreme Court has cautioned judges against applying this rule of lenity very often. As the 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.’”

The dispute over bump stocks also raises a perennial issue in gun policy. Congress wrote a statute nearly four decades ago that sought to ban machine guns. But that statute had to define the term machine gun in some way. And gun manufacturers eventually figured out how to design a device that will mimic a machine gun, but that arguably does not fall within the statutory definition that Congress wrote so many years ago.

This problem of imperfect definitions and lawyered-up gun makers was recently before the Supreme Court in Garland v. Blackhawk Manufacturing Group, a case involving “ghost guns.” In that case, gun manufacturers attempted to evade a federal law requiring background checks for gun buyers, and requiring guns to have serial numbers, by selling guns that had to be assembled much like an Ikea desk. The gun manufacturers then claimed that these unfinished guns did not fall within the federal definition of a “firearm.”

In any event, the Supreme Court ruled against the gun makers behind these ghost guns. So that is, perhaps, a hopeful sign for proponents of the bump stock ban. But, as a matter of statutory construction, the question of whether a gun fitted with a bump stock counts as a machine gun is genuinely difficult — difficult enough that the justices could write a plausible-sounding opinion ruling in favor of either party in this case.

If the Supreme Court legalizes bump stocks, Congress may never be able to ban them again

One other problem looming over this case is that, in New York State Rifle & Pistol Association v. Bruen (2022), the Supreme Court said that “the Second Amendment protects the possession and use of weapons that are ‘in common use at the time.’” So, as a weapon becomes more common and widely available, it gains greater constitutional protection. A weapon that is banned today could become constitutionally protected tomorrow if enough gun owners obtain one.

That creates a serious danger if the Supreme Court does determine that bump stocks are legal. Congress could potentially enact a new law explicitly banning bump stocks. But, by the time that law takes effect, many individual gun owners may have purchased a bump stock. And that could render the new bump stock ban unconstitutional.

Indeed, Bruen creates a perverse incentive for both gun manufactures and gun owners. If a new, extremely dangerous weapon is invented tomorrow — or if an existing, equally dangerous weapon is briefly made legal tomorrow — people who want to expand the scope of the Second Amendment have an incentive to distribute that weapon as quickly as possible.

Because once that weapon is “in common use,” it can no longer be banned.