Supreme Court Rejects Trump-Era Ban on Gun Bump Stocks

The Supreme Court on Friday struck down a ban on bump stocks, which enable semiautomatic rifles to fire at speeds rivaling those of machine guns, erasing one of the government’s rare firearm regulations to result from a mass shooting.

The decision, by a vote of 6 to 3, split along ideological lines. Justice Clarence Thomas, writing for the majority, said that the Bureau of Alcohol, Tobacco, Firearms and Explosives had exceeded its power when it prohibited the device by issuing a rule that classified bump stocks as machine guns.

“We hold that a semiautomatic rifle equipped with a bump stock is not a ‘machine gun’ because it cannot fire more than one shot ‘by a single function of the trigger,’” Justice Thomas wrote. His opinion included several diagrams of the firing mechanism, and he described in technical detail the internal workings of a firearm to show how a bump stock works.

The Trump administration enacted the ban after a gunman opened fire at a Las Vegas concert in 2017, one of the deadliest massacres in modern American history.

The decision was a forceful rejection of one of the government’s few steps to address gun violence, particularly as legislative efforts have stalled in Congress. It also highlighted the deep divisions on the court as the country continues to grapple with mass shootings.

The narrowly written decision was not a Second Amendment challenge. Rather, it is one of several cases this term seeking to undercut the power of administrative agencies. The court has yet to issue many of those opinions, including a challenge to a seminal precedent known as Chevron. However, the bump stock decision could signal support among the conservative justices for curbing the authority of administrative agencies.

Justice Sonia Sotomayor dissented, joined by Justices Elena Kagan and Ketanji Brown Jackson.

Justice Sotomayor summarized her dissent from the bench, a practice reserved for profound disagreements and the first such announcement of the term. “The majority puts machine guns back in civilian hands,” she said.

“When I see a bird that walks like a duck, swims like a duck, and quacks like a duck, I call that bird a duck,” Justice Sotomayor wrote. “A bump-stock-equipped semiautomatic rifle fires ‘automatically more than one shot, without manual reloading, by a single function of the trigger.’ Because I, like Congress, call that a machine gun, I respectfully dissent.”

Under the National Firearms Act of 1934, Congress outlawed machine guns, defined as “any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger.” That definition was expanded under the Gun Control Act of 1968 to include parts that can be used to convert a weapon into a machine gun, a category heavily regulated by the A.T.F.

A bump stock frees the weapon to slide back and forth rapidly, harnessing the energy from the kickback shooters feel when the weapon fires. The justices appeared divided over whether that counted as one pull of the trigger or multiple.

Until the Trump administration enacted its ban, bump stocks were considered legal; under an earlier interpretation of the law, they were found to increase the speed of a gun by sliding the stock back and forth to rapidly pull the trigger, not by “a single function of the trigger” as required for a machine gun.

The decision prompted immediate blowback. Democrats seized on it, blaming former President Donald J. Trump and saying that both his actions and his nominees on the court were decisive factors in the outcome.

President Biden, in a statement, urged Congress to act to ban the device. “Americans should not have to live in fear of this mass devastation,” he said.

Mr. Trump said in a statement that the court’s decision should be respected. Discussing gun rights in a speech on Friday night, he made no direct acknowledgment of the decision or his part in the ban, vowing only to “fully uphold” the Second Amendment, which he called “so important.”

The man who challenged the bump stock ban is Michael Cargill, a gun shop owner in Texas, backed by the New Civil Liberties Alliance, an advocacy group with financial ties to Charles Koch, a billionaire who has long supported conservative and libertarian causes. The organization primarily targets what it considers unlawful uses of administrative power.

Mr. Cargill said that he was working in bed on his iPad when he refreshed the Supreme Court’s website and saw the decision. He said he was so overwhelmed that he tumbled out of bed. “I was ecstatic,” he said. “For the first time in my life, I was at a loss for words.”

The case was a broader victory for gun rights, he said, adding that he believed it would make it easier to challenge future attempts by the A.T.F. to regulate firearms.

Justice Thomas said that the dissenting justices ignored Congress’s definition of a machine gun.

“A bump stock does not convert a semiautomatic rifle into a machine gun any more than a shooter with a lightning-fast trigger finger does,” Justice Thomas wrote.

In his concurring opinion, Justice Samuel A. Alito Jr. largely agreed with that assessment, finding that “there is simply no other way to read the statutory language.”

“The horrible shooting spree in Las Vegas in 2017 did not change the statutory text or its meaning,” Justice Alito wrote. “There is a simple remedy for the disparate treatment of bump stocks and machine guns. Congress can amend the law.”

In her dissent, Justice Sotomayor pushed back on those interpretations.

“Today, the court puts bump stocks back in civilian hands,” Justice Sotomayor wrote. “To do so, it casts aside Congress’s definition of ‘machine gun’ and seizes upon one that is inconsistent with the ordinary meaning of the statutory text and unsupported by context or purpose.”

“This is not a hard case,” she wrote. She added that the majority opinion “looks to the internal mechanism that initiates fire, rather than the human act of the shooter’s initial pull,” in an interpretation that “requires six diagrams and an animation to decipher the meaning of the statutory text.”

The lethal power of the device came into startling view in October 2017.

That month, a high-stakes gambler, Stephen Paddock, 64, perched on the 32nd floor of a Las Vegas hotel, opened fire on a country music festival, killing 60 people and injuring hundreds. In about 11 minutes, he fired more than 1,000 rounds of ammunition. In his arsenal were about a dozen AR-15-style rifles outfitted with the device.

Political pressure to act began to build, intensifying after a mass shooting at a school in Parkland, Fla. Mr. Trump vowed to ban the device.

Justice Department officials initially said the executive branch could not ban bump stocks without action by Congress, before ultimately reversing course. Under the ban, possession or sale of bump stocks could lead to prison time.

Divisions in the lower federal courts increased the likelihood that the Supreme Court would weigh in.

After a federal trial judge in Texas sided against Mr. Cargill, he appealed to the U.S. Court of Appeals for the Fifth Circuit. Eventually, the full court, one of the country’s most conservative appeals courts, agreed with Mr. Cargill, split along ideological lines.

“A plain reading of the statutory language, paired with close consideration of the mechanics of a semiautomatic firearm, reveals that a bump stock is excluded from the technical definition of ‘machine gun’ set forth in the Gun Control Act and National Firearms Act,” Judge Jennifer Walker Elrod, a George W. Bush appointee, wrote.

The three dissenting judges, all Democratic appointees, argued that the majority’s reasoning served to “legalize an instrument of mass murder.”

Adam Liptak and Michael Gold contributed reporting.