The US Supreme Court has ruled 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 favour 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 re-engage 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.”President Joe Biden said the decision “strikes down an important gun safety regulation”.“Americans should not have to live in fear of this mass devastation,” he added, saying he has “used every tool in my administration to stamp out gun violence”.“I call on Congress to 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,” Biden said.Trump campaign spokesperson Karoline Leavitt said after the ruling, that “the court has spoken and their decision should be respected” and called him a “fierce defender” of gun rights.The ruling sparked howls of outrage from Democrats, with Biden’s re-election campaign denouncing the court for prioritising the gun lobby over “the safety of our kids”.“Weapons of war have no place on the streets of America, but Trump’s Supreme Court justices have decided the gun lobby is more important than the safety of our kids and our communities,” campaign spokesman Michael Tyler said in a statement.Hakeem Jeffries, the Democratic minority leader in the House of Representatives, called the ruling “dangerous, disastrous and deeply disturbing” while Dick Durbin, chairman of the powerful Judiciary Committee in the Democratic-led Senate, called it “deeply disappointing”.“By undoing this policy, the court is putting countless American lives at risk,” Jeffries said.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.However, lawyers for Michael Cargill, a gun seller from Texas, challenged the move, claiming that 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 that the law defines a machine gun strictly as a weapon capable of firing “automatically more than one shot ... by a single function of the trigger”.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 favour stricter gun regulations, but the powerful firearms lobby and mobilised voters supporting America’s strong firearms culture have hindered congressional action.Mark Chenoweth, president of the conservative legal group New Civil Liberties Alliance that represented Cargill, hailed the ruling.“The statute Congress passed did not ban bump stocks, and the ATF does not have the power to do so on its own,” Chenoweth said.John Feinblatt, president of the gun control advocacy group Everytown for Gun Safety, urged Congress to outlaw bump stocks.“Guns outfitted with bump stocks fire like machine guns, they kill like machine guns, and they should be banned like machine guns – but the Supreme Court just decided to put these deadly devices back on the market,” Feinblatt said.