The Supreme Court on Friday struck down a ban on bump stocks enacted by the Trump administration after a deadly mass shooting in Las Vegas in 2017.
The decision, by a vote of 6 to 3, split along ideological lines. Justice Clarence Thomas, writing for the majority, found that the Bureau of Alcohol, Tobacco, Firearms and Explosives had exceeded its power when it prohibited the device, an attachment that enables a semiautomatic rifle to fire at a speed rivaling that of a machine gun.
The agency, he added, had overstepped in 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. He included several diagrams of the firing mechanism in the opinion.
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 over the issue as the country grapples with gun violence.
Although the case centers on firearms, it is not a Second Amendment challenge. Rather, it is one of several cases this term seeking to undercut the power of administrative agencies.
Justice Sonia Sotomayor filed a dissent, 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.”
The challenger was 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.
Mark Chenoweth, the president of the legal advocacy group, said in statement that the organization was “delighted” that the court had sided with Mr. Cargill.
“This result is completely consistent with the Constitution’s assignment of all legislative power to Congress,” Mr. Chenoweth said. “Bump-stock opponents should direct any views at Congress, not the court, which faithfully applied the statute in front of it.”
During oral arguments in February, the debate seemed to center on whether the device transformed a firearm into a “machine gun.” That would enable the accessory to be prohibited as part of a category that is heavily regulated by the A.T.F.
The justices appeared to struggle to understand the mechanics of a bump stock and precisely how it increases a gun’s firing speed.
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.
Until the Trump administration enacted its ban, bump stocks were considered legal; under an earlier interpretation of the law, they 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 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 hotel in Las Vegas and opened fire on a country music festival, killing 60 people and injuring hundreds. In his arsenal were a dozen AR-15-style rifles outfitted with the device.
In about 11 minutes, he fired more than 1,000 rounds of ammunition in what remains the deadliest mass shooting in modern U.S. history. Investigators later said they found about a dozen rifles modified with bump stocks in his hotel suite.
Political pressure to do something about the device immediately began to build. Legislators, including several Republicans, suggested they may be open to outlawing bump stocks. The National Rifle Association, in an unusual move, endorsed tighter restrictions.
President Donald J. Trump, a vocal supporter of the Second Amendment, promised to ban the device.
Justice Department officials agreed to a review of the legality of bump stocks, but A.T.F. officials had privately indicated that banning the device would most likely require action by Congress, a tough fight on any gun regulation.
In December 2018, the Trump administration announced a ban. It gave gun owners until late March 2019 to turn over or destroy bump stocks.
Under the ban, possession or sale of bump stocks could lead to prison time.
Adam Liptak contributed reporting.
