The Supreme Court on Friday struck down a Trump-era ban on bump stocks. A bump stock is a gun accessory that allows semi-automatic weapons to fire rapidly like a machine gun and that was used in some of the deadliest mass shootings in modern US history.

The high court found 6-3 that the Trump administration did not follow federal law when it reversed its decision and banned bump stocks after a gunman attacked a country music festival in Las Vegas with assault rifles in 2017. He fired more than 1,000 rounds into the crowd in 11 minutes, killing 60 people and wounding hundreds.

A Texas gun shop owner challenged the ban, arguing that the Justice Department wrongly classified the accessories as illegal machine guns.

The Biden administration said the Bureau of Alcohol, Tobacco, Firearms and Explosives made the right choices for accessories that can enable weapons to fire at a rate of hundreds of rounds per minute.

It was the latest gun case to come before the high court, where a conservative majority handed down a landmark ruling expanding gun rights in 2022 and another gun case challenging a federal law aimed at keeping guns away from people under domestic violence restraining orders is also being considered.

However, the arguments in the bump stock case were more about the Second Amendment than whether the ATF had overstepped its authority.

Justices on the court’s liberal wing suggested that it is “common knowledge” that anything capable of firing a “volley of bullets” is a machine gun under federal law. However, conservative justices raised questions about why Congress has not taken action to ban bump stocks, as well as the implications of the ATF changing its mind a decade after declaring the accessories legal.

The high court took over the case after lower courts differed over bump stocks, which were invented in the early 2000s. Under Republican Presidents George W. Bush and Democrat Barack Obama, the ATF ruled that bump stocks do not turn semiautomatic weapons into machine guns. The agency reversed those rulings at Trump’s urging after the Las Vegas shooting and another mass shooting at Parkland High School in Florida, which killed 17 people.

Bump stocks are accessories that replace the rifle’s stock, the part that rests on the shoulder. They use the gun’s recoil energy to make the trigger hit the shooter’s stationary finger, allowing the gun to fire at a rate comparable to that of a conventional machine gun. Fifteen states and the District of Columbia have their own restrictions on bump stocks.

The plaintiff, Texas gun shop owner and military veteran Michael Cargill, was represented by the New Civil Liberties Alliance, a group funded by conservative donors such as the Koch network. His lawyers acknowledged that bump stocks allow for faster fire, but argued they are different because the shooter has to exert more effort to keep the gun firing.

Government lawyers said the effort expected from the shooter was small and would have made no legal difference. The Justice Department said the ATF changed its view on bump stocks and came to the right conclusion after conducting a more thorough investigation prompted by the Las Vegas shooting.

The plaintiffs said in court documents that when the ban went into effect in 2019, about 520,000 bump stocks were in circulation, causing people to either surrender them or destroy them, leading to an estimated total loss of $100 million.