Washington DC - The US Supreme Court on Wednesday is hearing arguments on the legality of bump stocks, simple devices that can allow automatic fire from otherwise semi-automatic guns.

The case stems from the worst mass shooting in US history, in October 2017, when a man – using guns equipped with bump stocks – fired on a crowd attending an outdoor music concert in Las Vegas, killing 58 people and wounding around 500.

In response to the deadliest ever mass shooting committed by a single gunman in the US, the Bureau of Alcohol, Tobacco and Firearms moved to revise its regulations on bump stocks, declaring them in December 2018 as falling under an existing ban on machine guns, making their possession a crime.

The Justice Department under Donald Trump's administration had worked with the ATF to declare detachable devices illegal.

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Another shocking mass shooting had occurred that year at a high school in Parkland, Florida, leaving 17 dead.

But the ATF rule was challenged in court almost immediately, and has now worked its way up to the Supreme Court in a case pitting President Joe Biden's Justice Department against Michael Cargill, a gun seller from Texas.

"A bump stock transforms a semiautomatic rifle into a weapon that shoots hundreds of bullets per minute with a single pull of the trigger," US Solicitor General Elizabeth Prelogar said in written arguments.

Cargill's lawyers argue that the ATF overstepped its bounds, and that classifying bump stocks as machine guns "is a decision for Congress to make, not agencies or courts."

The Supreme Court previously expanded gun rights in a 2022 ruling that said Americans have a fundamental right to carry a handgun in public, though certain regulations still exist around the practice.

A conservative federal appeals court ruled in March 2023 that a law barring people under domestic violence court orders from owning a gun was unconstitutional.

That case will also be decided by the Supreme Court this year. The justices seemed likely to uphold the domestic violence law when they heard arguments in November.

Polls show a majority of Americans favor stricter gun regulations, but the powerful firearms lobby led by the NRA and Republican opposition has made congressional action difficult.