The US Supreme Court ruled on Friday that a ban introduced by ex-president Donald Trump's administration on bump stocks -- devices which allow semi-automatic rifles to fire like a machine gun -- is unconstitutional.
The case stems from the worst mass shooting in US history, in October 2017, when a man fired on a crowd attending an outdoor music concert in Las Vegas, killing 58 people and wounding around 500.
Most of his 22 guns were equipped with bump stocks, allowing them to fire as many as nine bullets a second.
The court voted along ideological lines, 6-3 in favor of the conservative justices, that the Trump administration did not follow the law after the shooting in extending a ban on machine guns to include bump stocks.
"This case asks whether a bump stock -- an accessory for a semiautomatic rifle that allows the shooter to rapidly reengage the trigger (and therefore achieve a high rate of fire) -- converts the rifle into a 'machinegun,'" said Justice Clarence Thomas, writing the opinion for the majority.
"We hold that it does not."
The government first acted on the issue in February 2018, following another mass shooting at a Florida high school which left 17 people dead, when the Justice Department under Trump moved to declare the detachable devices illegal.
In December of that year, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) revised its regulations on bump stocks, declaring that they fall under a 1934 law passed by Congress banning machine guns.
Brian Fletcher, deputy solicitor general in President Joe Biden's Justice Department, told the court when it heard oral arguments in February that bump stocks allow a user to "empty a 100-round magazine like the ones used in the Las Vegas shooting in about 10 seconds."
"Those weapons do exactly what Congress meant to prohibit when it enacted the prohibition on machine guns," Fletcher said.
But lawyers for Michael Cargill, a gun seller from Texas, challenged the move claiming the ATF had overstepped its bounds in classifying bump stocks with machine guns.
Oral arguments focused on the technical definition of a machine gun in the 1934 law, which was passed during the Prohibition era, well before the invention of the bump stock.
Thomas said in his opinion the law defines a machine gun as any weapon capable of firing "automatically more than one shot... by a single function of the trigger."
"We hold that a semiautomatic rifle equipped with a bump stock is not a 'machinegun' because it cannot fire more than one shot 'by a single function of the trigger.'
"And, even if it could, it would not do so 'automatically.' ATF therefore exceeded its statutory authority by issuing a Rule that classifies bump stocks as machineguns."
Justice Samuel Alito, a conservative and fierce defender of the Second Amendment, which guarantees Americans the right to own guns, said in a concurring opinion that it was up to Congress to decide if the 1934 law should apply to the devices.
"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," he said.
But the ruling prompted a robust dissent from liberal Justice Sonia Sotomayor.
"Today, the Court puts bump stocks back in civilian hands. To do so, it casts aside Congress's definition of 'machinegun' and seizes upon one that is inconsistent with the ordinary meaning of the statutory text and unsupported by context or purpose," she wrote.
"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."
Polls show a majority of Americans favor stricter gun regulations, but the powerful firearms lobby and mobilized voters supporting America's strong firearms culture have hindered congressional action.
