The Supreme Court threw out a Trump-era rule that banned so-called “bump stocks” that allow rifles to mimic automatic fire, finding the government overstepped its authority when it relied on a federal law that applies to machine guns.

In a sharply divided 6-3 decision, the conservative justices in the majority ruled that the Bureau of Alcohol, Tobacco, Firearms and Explosives could not ban the device that attaches to the back of a firearm that allows it to mimic automatic weapons.

Justice Samuel A. Alito Jr. wrote separately to point out that it is up to Congress to prohibit bump stocks such as the one used in the “horrible shooting spree” in Las Vegas that killed more than 50 people at a music festival in 2017.

“There is a simple remedy for the disparate treatment of bump stocks and machineguns. Congress can amend the law — and perhaps would have done so already if ATF had stuck with its earlier interpretation,” Alito wrote. “Now that the situation is clear, Congress can act.”

The majority opinion, written by Justice Clarence Thomas, said that adding the bump stock to the back of the weapon did not change the specifics of the internal trigger mechanism, which is what the law originally written in 1934 focused on when it banned machine guns.

“We conclude that a semiautomatic rifle equipped with a bump stock is not a ‘machinegun’ because it does not fire more than one shot ‘by a single function of the trigger,’” Thomas wrote.

Justice Sonia Sotomayor, in a dissent joined by Justices Elena Kagan and Ketanji Brown Jackson, wrote that the decision “will have deadly consequences.”

Sotomayor, who read a portion of her dissent from the bench, criticized the majority for casting aside congressional intent in the federal law that bans weapons that fire “automatically more than one shot, without manual reloading, by a single function of the trigger.”

“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,” Sotomayor wrote. “Because I, like Congress, call that a machinegun, I respectfully dissent.”

Sotomayor wrote that the decision will put “bump stocks back in civilian hands” and invites gunmakers to create new devices that mimick automatic rifles.

“The majority’s artificially narrow definition hamstrings the Government’s efforts to keep machineguns from men like the Las Vegas shooter,” Sotomayor wrote.

A bump stock is an add-on to a gun that uses recoil to “bump” the gun back into the trigger finger of the shooter, firing faster than usual. Congressional leaders declined to act on bipartisan legislation introduced after the Las Vegas shooting.

President Donald Trump ordered the government to come up with a bump stock ban in 2018, as students lobbied state and federal lawmakers to pursue regulation after 17 people were killed by a gunman at a school in Parkland, Fla.

The ATF issued the rule restricting the devices by defining them as machine guns, which are restricted under long-standing federal law.

The case came to the court after Michael Cargill surrendered several of the devices and challenged the rule in a Texas federal court. Cargill said the ATF had correctly interpreted the federal law for years when it had approved the devices before 2017.

The U.S. Court of Appeals for the 5th Circuit nixed the rule for going beyond the definition of “machine gun” in the law, writing that the definition hinged on a single operation of the trigger mechanism.

The Biden administration then asked the justices to intervene, arguing that the law was meant to hinge on what a shooter does rather than the specifics of a trigger mechanism. Adopting the narrower approach, the administration said, would invite creative methods to duplicate machine guns.

Thomas largely endorsed the 5th Circuit’s approach in Friday’s opinion, replete with illustrations about the function of a rifle’s trigger mechanism.

Calls for legislation

President Joe Biden, in a statement Friday that described the bump stock ban as “an important gun safety regulation,” did not indicate he would to try using executive powers to ban them. Rather, Biden said Congress should “ban bump stocks, pass an assault weapon ban and take additional action to save lives — send me a bill and I will sign it immediately.”

In 2022, Democrats in the House passed a bill that included a bump stock ban, among other measures, in response to several mass shootings. However, the ban was not included in a broader compromise bill on gun violence that Congress passed later that year.

Democrats roundly criticized the Supreme Court’s decision and called for Congress to pass legislation addressing bump stocks. In a statement, Sen. Catherine Cortez Masto, D-Nev., said the devices “have no place on our streets.”

Rep. Dina Titus, D-Nev., who represents part of Las Vegas, said in a statement Friday that she was “appalled” by the court’s decision and called on Congress to pass legislation she sponsored that would codify a ban on bump stocks.

“This ruling hits home for many communities that have been impacted by gun violence caused by rapid fire gun accessories, including District One,” Titus said, referring to her congressional district.

Senate Judiciary Chair Sen. Richard J. Durbin, D-Ill., criticized the ruling in a statement and said he would work with any Republicans willing to negotiate new gun control laws.

Legislation on guns would have to navigate a Republican-controlled House and a closely divided Senate where trust in the Biden administration is low amid election year politics.

John T. Bennett contributed to this report.