The US Supreme Court lifted the ban on gun bump stocks.
Bump stocks were used in the Las Vegas shooting in 2017, which murdered dozens of people at a music festival.
The US Supreme Court removed the restriction on bump stocks, the rapid-fire gun attachment used in America's bloodiest mass shooting.
The court ruled on Friday that the government did not have the authority to outlaw the accessories.
Bump stocks were outlawed by the Trump administration following their usage in a 2017 shooting that murdered 60 people at a Las Vegas concert.
However, a Texas gun shop owner who challenged the prohibition contended the government went too far in categorizing the accessories as machine guns, which are mainly unlawful under federal law (with few exceptions), and brought his argument all the way to America's highest court.
US law forbids the transfer or possession of machine weapons manufactured after May 19, 1986, the effective prohibition date. However, the transfer or possession of machine guns lawfully obtained prior to that date is permitted.
The court ruled that a semi-automatic rifle with an attachment does not qualify as a machine gun under federal law.
Conservative Justice Clarence Thomas wrote the Supreme Court judgment, which stated that the Bureau of Alcohol, Tobacco, and Firearms had "exceeded" its power.
The court stated that rifles with a bump stock "cannot fire more than one shot 'by a single function of the trigger', and even if they could, they would not do so 'automatically'" (citing a portion of the legal definition of machine guns).
° What are bump stock?
The divided ruling saw three of the nine judges dissent - they were judges Ketanji Brown Jackson, Elena Kagan and Sonia Sotomayor.
Justice Sotomayor said: "Today, the Court puts bump stocks back in civilian hands.
" A decision which she said "will have deadly consequences".
Referencing whether they should qualify as machine guns, she said: "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.
" In the 1986 Firearms Act, machine guns are described as any "weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual reloading, by a single function of the trigger".
During a March hearing on the issue, four conservative-led justices questioned the restriction, pointing out slight technical distinctions between how a bump-stock pistol fires and a machine gun.
At the time, Justice Neil Gorsuch stated that while he could understand "why these items should be made illegal," it was Congress's responsibility to do so clearly.
Justice Ketanji Brown Jackson countered, stating that bump stocks are simply "the kinds of weapons Congress intended to prohibit because of the harm they cause."
The bump stock uses a rifle's recoil to quickly fire numerous bullets. It substitutes the weapon's stock, which is held against the shoulder, allowing the gun to move back and forth between the user's shoulder and trigger finger.
That action, or bump, allows the gun to fire without the user moving their finger. The Las Vegas shooter had added bump stocks to 12 of his semi-automatic rifles, allowing him to fire hundreds of rounds per minute, comparable to numerous machine guns. He killed 60 people and wounded hundreds more at a music festival.
A spokeswoman for Donald Trump's campaign, whose administration enacted the original ban, told the BBC that "the court has spoken and their decision should be respected".
A spokesman for President Joe Biden, who is due to confront Trump in June as the two candidates compete for reelection, criticized the decision. "Weapons of war have no place on the streets of America," they declared.
