How gun accessories called bump stocks ended up before the U.S. Supreme Court

When gun accessories known as bump stocks first hit the market more than a decade ago, the U.S. 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 U.S. history.

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

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