The U.S. Supreme Court has struck down a Trump-era regulation that effectively banned bump stocks, aftermarket accessories that make semiautomatic rifles fire more like machine guns. The devices were used in the deadliest mass shooting in American history.

The justices split along ideological lines. A majority of six found that the definition of machine gun in federal law does not apply to bump stocks. As a result, the Bureau of Alcohol, Tobacco, Firearms and Explosives exceeded its authority to regulate them, the court ruled.

“A bump stock does not convert a semiautomatic rifle into a machinegun any more than a shooter with a lightning-fast trigger finger does,” Justice Clarence Thomas wrote in the majority opinion.

The decision, Garland v. Cargill, comes almost exactly two years after the court — emboldened by three conservative justices appointed by former President Donald Trump — expanded gun rights in New York State Rifle and Pistol Association v. Bruen in 2022. But this case had little to do with the Second Amendment or Bruen. Cargill and his attorneys did not argue that the Second Amendment protects bump stocks or machine guns. Instead, the technicalities of bump stocks and the legal definition of machine gun dominated arguments.

Machine guns — which continue to fire bullets as long as the trigger is held down — have been strictly regulated since 1934, when Congress passed the National Firearms Act. Informally known as the “Anti-Machine Gun Bill” when it was proposed, the law required people to pay a tax and register their machine gun in order to possess one. 

Decades later, the Firearm Owners’ Protection Act of 1986 banned the civilian possession and transfer of newly manufactured machine guns. Today, possession of such guns is a federal crime, with a penalty of up to 10 years in prison.

Semiautomatic weapons, which fire one bullet per trigger pull, are legal and don’t need to be registered with the federal government. When a bump stock is employed, it uses a semiautomatic’s natural recoil to quickly re-engage the trigger as long as the shooter maintains pressure. That enables an increased rate of fire — one that can nearly match that of a machine gun.

For years, the ATF did not regulate bump stocks. That changed after the Route 91 Harvest Festival shooting in Las Vegas in 2017. The shooter, armed with a dozen rifles equipped with bump stocks, killed 60 people. More than 400 people were wounded by gunfire, and hundreds more were injured in the ensuing panic.

Amid mounting public pressure, the Trump administration’s ATF reversed course and effectively banned the devices by defining them as machine guns under the National Firearms Act. The agency directed bump stock owners to detach and dispose of the devices or face criminal penalties. In 2019, the Department of Justice estimated that as many as 520,000 bump stocks were in circulation. 

Michael Cargill, a Texas gun rights advocate, complied with the regulations and turned over several bump stocks. He then sued the ATF, arguing that the agency erred in its interpretation and lacked the authority to make such a determination in the first place.

His challenge made its way to the 5th Circuit Court of Appeals, where a three-judge panel unanimously upheld the regulation in 2021. The full court later agreed to hear the case and, in a split decision, ruled against the ATF.

The 5th Circuit’s decision came after three other appeals courts upheld the rule, creating a split that invited a Supreme Court intervention. The Biden administration appealed, and the justices heard oral arguments in February.

During oral arguments, some of the court’s conservative justices appeared sympathetic to the regulation, while also hinting at some discomfort with the ATF’s authority.

“I am entirely sympathetic to your argument, and it seems like, yes, that this is functioning like a machine gun would,” Justice Amy Coney Barrett said while questioning the Biden administration’s deputy solicitor general. “But looking at that definition, I think the question is, why didn’t Congress pass that legislation?”

Barrett ultimately joined the court’s majority opinion, though, which focused on the technicalities of bump stocks, going so far as to include diagrams of bump stocks and firearms. The largest area of disagreement centered around whether the federal statute intended to focus on the shooter’s pull of the trigger, or the mechanical functioning of the firearm itself. 

“A bump stock does not alter the basic mechanics of bump firing,” Thomas wrote for the majority. ”As with any semiautomatic firearm, the trigger still must be released and reengaged to fire each additional shot.”

The court’s liberal justices signed onto a dissent penned by Justice Sonia Sotomayor, which panned the majority’s reasoning: “This is not a hard case. All of the textual evidence points to the same interpretation. A bump-stock-equipped semiautomatic rifle is a machinegun because (1) with a single pull of the trigger, a shooter can (2) fire continuous shots without any human input beyond maintaining forward pressure.”

Cargill was not as great a blow to the ATF as it could have been. In recent years, the Supreme Court has been considering whether to limit federal agencies’ authority to interpret federal law and Congress’s ability to delegate policymaking to agencies.

“They sidestepped that question completely,” said Dru Stevenson, a law professor at the South Texas College of Law in Houston. “They did not go all the way and say that the ATF has no authority to issue regulations, period.” 

But many of the ATF’s regulations rely on similar interpretations of federal law. The decision could have implications for the agency’s ability to regulate other weapons, components, and accessories, including pistol braces and ghost guns.

“The Supreme Court majority here interpreted statutory language very narrowly when it comes to defining what counts as different types of firearms,” said Eric Ruben, an associate professor of law at Southern Methodist University Dedman School of Law. “That wouldn’t bode well for other regulations that are focused on accessories and saying accessories transform one gun into another.”

But the opinion could buoy the regulation of auto sears, commonly referred to as Glock switches. The ATF has also said that those devices transform a semiautomatic weapon into a machine gun. In a footnote, the majority opinion seemed to endorse that finding.

Seventeen states and the District of Columbia have their own bump stock bans. Since the Supreme Court struck down the federal ban based solely on the ATF’s rulemaking authority and interpretation of federal statutes, the state bans will remain in effect, and Congress could act to ban bump stocks nationwide.

“This whole litigation is a reflection of political dysfunction when it comes to gun law,” Ruben said. “At the end of day, it’s not a Second Amendment opinion. There’s nothing in there that says that bump stocks can’t be banned. It’s going to be very telling whether Congress can quickly act to ban bump stocks.” 

The Cargill case is the first of two major gun cases the Supreme Court is expected to rule on this month. The other is United States v. Rahimi, a challenge to a federal law that prohibits the possession of guns by people under a domestic violence restraining order.