Fitting a bump stock to a rifle enables the weapon to fire hundreds of bullets per minute.

The Trump administration banned the devices by classifying them as machine guns after they were used in the deadliest mass shooting in US history.

Under the 1986 National Firearms Act, owning a machine gun is illegal.

But a Texas resident and gun shop owner Michael Cargill has challenged the ban on bump stocks, saying the government has interpreted what qualifies as a machine gun too broadly.

The case has now reached America's highest court.

It is illegal to modify the internal components of semi-automatic rifles - which typically manage about 60 aimed shots per minute - to make them fully automatic, but gun owners can legally buy accessories to increase the rate of fire.

The bump stock harnesses a rifle's recoil to rapidly fire multiple rounds. It replaces the weapon's stock, which is held against the shoulder, and allows the gun to slide back and forward between the user's shoulder and trigger finger. That motion - or "bump"- lets the gun fire without the user having to move their trigger finger.

At Wednesday's hearing, both liberal and conservative justices seemed to struggle with some of the more technical aspects of the case - about the function and application of bump stocks, as well as the statutes that define and ban machine guns.

Speaking to Mr Cargill's lawyer, liberal justice Elena Kagan asked how a gun with a bump stock could be differentiated from a machine gun.
She said a machine gun needed continued pressure on a trigger, while a bump stock needed continuous pressure on a barrel, adding: "I can't understand how anybody could think that those two things should be treated differently."

But conservative Justice Brett Kavanaugh pointed out that the two presidential administrations before Donald Trump's had found that rifles with bump stocks were not considered machine guns under the relevant federal statute. "That's reason for pause," Justice Kavanaugh said.

Another conservative justice, Amy Coney Barrett, highlighted another problem for the Biden administration, which has found itself defending the Trump-era ban: that Congress had not acted to ban bump stocks itself.

"Intuitively, I am entirely sympathetic to your argument. I mean, it seems like, yes, this is functioning like a machine gun would," she said. But she asked why Congress had not acted "to make this covered more clearly".

Wednesday's case is linked to a 2017 mass shooting, when a gunman opened fire at a concert in Las Vegas, killing 60 people and injuring hundreds. It was the deadliest shooting the country has ever experienced.

Officials determined that the attacker had attached bump stocks - which were then legal but controversial - onto 12 of his semi-automatic rifles to fire hundreds of rounds per minute, the same rate as many machine guns.

Political pressure grew to outlaw the relatively new devices, which could be obtained without the extensive background checks required for purchasing automatic weapons.

Shortly after the Las Vegas shooting, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) under President Donald Trump expanded the definition of machine gun to prohibit the production, sale, and possession of bump stocks.

In the 1986 firearms act, machine guns are defined 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".

After the ban was imposed, Texan Michael Cargill challenged the ATF's rule.

A federal appeals court reversed an earlier decision that "bump stocks qualify as machine guns under the best interpretation", saying it was too broad.

Now the conservative-majority Supreme Court will have the final word. Its ruling is expected in June.

If it sides with the Texas Fifth Circuit Court of Appeals, it would invalidate the ATF's rule and any states that do not have their own laws in place restricting bump stocks would automatically see bump stocks legalised again.