Washington, Feb 28 (AP) When gun accessories known as bump stocks first hit the market more than a decade ago, the US government initially concluded the devices did not violate federal law despite enabling semi-automatic weapons to spray bullets at the rapid-fire rate of a machine gun.

That changed after a gunman with bump stock-equipped rifles killed 60 people and wounded hundreds more at a Las Vegas music festival in 2017. It was the deadliest mass shooting in modern US history.

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Now a federal ban on bump stocks imposed under then-President Donald Trump is before the US Supreme Court, which heard arguments on Wednesday in a case that tests the limits of the government's ability to regulate guns in an era of mass shootings.

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Here's what to know about the case:

WHAT ARE BUMP STOCKS?

Bump stocks are accessories that replace a rifle's stock that gets pressed against the shoulder. When a person fires a semi-automatic weapon fitted with a bump stock, it uses the gun's recoil energy to rapidly and repeatedly bump the trigger against the shooter's index finger.

That allows the weapon to fire dozens of bullets in a matter of seconds.

Bump stocks were invented in the early 2000s after the expiration of a 1994 ban targeting assault weapons. The federal government approved the sale of bump stocks in 2010 after the Bureau of Alcohol, Tobacco and Firearms and Explosives concluded that guns equipped with the devices should not be considered illegal machine guns under federal law.

According to court documents, more than 520,000 bump stocks were in circulation by the time the Washington reversed course and imposed a ban that took effect in 2019.

WHY WERE BUMP STOCKS BANNED?

More than 22,000 people were attending a country music festival in Las Vegas on October 1, 2017, when a gunman opened fire on the crowd from the window of his high-rise hotel room.

It was the worst mass shooting in modern US history, leaving 60 people dead and injuring more than 850.

Authorities found an arsenal of 23 assault-style rifles in the shooter's hotel room, including 14 weapons fitted with bump stocks. Investigators later concluded the gunman, who killed himself before police reached his room, fired more than 1,000 rounds in just 11 minutes.

In the aftermath of the shootings, the ATF reconsidered whether bump stocks could be sold and owned legally. With support from Trump, the agency in 2018 ordered a ban on the devices, arguing they turned rifles into illegal machine guns.

Bump stock owners were given until March 2019 to surrender or destroy them.

WHO IS CHALLENGING THE BAN?

A group called the New Civil Liberties Alliance sued to challenge the bump stock ban on behalf of Michael Cargill, a Texas gun shop owner.

According to court records, Cargill bought two bump stocks in 2018 and then surrendered them once the federal ban took effect.

The case doesn't directly address the Second Amendment rights of gun owners. Instead, Cargill's attorneys argue that the ATF overstepped its authority by banning bump stocks. Mark Chenoweth, president of the New Civil Liberties Alliance, says his group wouldn't have sued if Congress had banned them by law.

HOW DID THE CASE END UP BEFORE THE SUPREME COURT?

The Supreme Court took up the case after lower federal courts delivered conflicting rulings on whether the ATF could ban bump stocks.

The ban survived challenges before the Cincinnati-based 6th US Circuit Court of Appeals, the Denver-based 10th Circuit, and the federal circuit court in Washington.

But the 5th US Circuit Court of Appeals based in New Orleans struck down the bump stock ban when it ruled in the Texas case last year. The court's majority in the 13-3 decision found that “a plain reading of the statutory language" showed that weapons fitted with bump stocks could not be regulated as machine guns.

The Supreme Court agreed to take up the case last November. (AP)

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