The US Supreme Court ruled on Friday that a ban introduced by ex-president Donald Trump's administration on "bump stocks" -- 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 took action on bump stocks 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 an existing 1934 law passed by Congress banning machine guns.

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.