The fate of a ban on bump stocks is in the hands of Congress after the Supreme Court ruled Friday that current law doesn’t allow the government to outlaw them as equivalent to machine guns.

The justices said in a 6-3 ruling that the Trump administration was wrong to interpret the federal prohibition of machine guns to extend to bump stocks but nothing stops Congress from changing the law to explicitly cover bump stocks.

Democrats were quick to take up the invitation.

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“It is really critical that we continue to make sure we can keep guns out of the hands of people who shouldn’t have them,” said Rep. Debbie Wasserman Schultz, Florida Democrat.

“Send me a bill, and I will sign it immediately,” President Biden said.

Conservative Republicans signaled that the idea would be met with resistance.

“It was a foolish mistake under both administrations. I think this was a Trump-era rule,” said Rep. Chip Roy, Texas Republican. “The court striking it down is a win for people’s Second Amendment rights.”

A bump stock is a device that attaches to the rear of a rifle, helping it quickly slide back and forth. The resulting motion helps repeatedly engage the trigger fast enough to simulate automatic fire.

Justice Clarence Thomas, writing the key opinion for the court, said that doesn’t match the law’s definition of an automatic weapon, which specifies “a single function of the trigger.”

He said a rifle still requires the right amount of pressure from the hand on the barrel to operate.

“Without this ongoing manual input, a semiautomatic rifle with a bump stock will not fire multiple shots. Thus, firing multiple shots requires engaging the trigger one time — and then some,” Justice Thomas wrote.

Justice Sonia Sotomayor, writing the dissent for the three Democratic appointees, said Justice Thomas was dissecting definitions too finely. She said bump stocks seem like machine guns to her.

“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,” she wrote.

For years, the Bureau of Alcohol, Tobacco, Firearms and Explosives saw the law like Justice Thomas did. That changed in 2018, a year after a gunman armed with rifles outfitted with bump stocks perpetrated the worst mass shooting in modern U.S. history.

From a perch in a Las Vegas hotel, he fired down at concertgoers, killing 58 people and wounding more than 500 others.

That massacre prompted the Trump administration’s rewrite.

Las Vegas also loomed large for the justices.

“All the shooter had to do was pull the trigger and press the gun forward. The bump stock did the rest,” Justice Sotomayor said.

Justice Samuel A. Alito Jr., who joined the majority, said tragedies can’t trump the law.

“The horrible shooting spree in Las Vegas in 2017 did not change the statutory text or its meaning,” he wrote. “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. Now that the situation is clear, Congress can act.”

The case was brought by Michael Cargill, owner of Central Texas Gun Works, who challenged ATF’s 2018 rule reversal. He argued that the ATF violated procedural law by attempting to change the policy without going to Congress.

The 5th U.S. Circuit Court of Appeals ruled in his favor, reasoning that the government’s use of a decades-old ban on machine guns and applying that to bump stocks was ambiguous.

The Supreme Court’s decision Friday affirmed that ruling.

“This is huge for the Second Amendment community. This is huge for everyone in America. If you’re pro-gun or anti-gun, it doesn’t matter. This is about the administrative agency,” Mr. Cargill told The Washington Times, adding an agency can’t write laws.

“It has to go back to Congress. Congress has to write the law,” he said.

That’s easier said than done.

Gun legislation is notoriously difficult to get through Congress, and chances are even tougher after the experience in 2022.

The Bipartisan Safer Communities Act was passed in the wake of mass shootings in Buffalo, New York, and Uvalde, Texas. It sought to tighten rules on who could purchase and sell firearms.

Earlier this year, Mr. Biden issued a policy, which he said would carry out the law, requiring background checks for far more firearms sales.

Republicans who worked on the 2022 bill said Mr. Biden betrayed them by stretching the law beyond what they intended. Sen. John Cornyn, a Texas Republican who was instrumental in the negotiations, said Mr. Biden had become “a dishonest broker.”

The relatively few days Congress is in session before the November elections means the gun issue will likely matter less in the legislative world and more in campaign trail politics.

• Kerry Picket and Lizzie Donker contributed to this report.