(Bloomberg) -- The US Supreme Court weighed dealing a fresh blow to gun regulations as the justices questioned the federal ban on bump stocks, attachments that let a semiautomatic rifle fire at speeds comparable to a machine gun.

Hearing arguments in Washington Wednesday, the court suggested it will divide over a criminal prohibition former President Donald Trump’s administration put in place after the 2017 Las Vegas concert massacre, when a man using bump stocks killed 60 people. The attack was the deadliest mass shooting in modern American history.

Justice Elena Kagan said bump stocks let shooters easily fire “a torrent of bullets,” putting the devices “in the heartland” of what Congress was seeking to prohibit when it outlawed machine guns in 1986.

But the court’s conservative majority had a more mixed reaction, leaving the likely outcome unclear. Justice Brett Kavanaugh said the ban would “ensnare a lot of people who are not aware of the legal prohibition.”

The case is one of two firearms disputes the court is considering in its 2023-24 term, along with a constitutional clash over the federal gun ban for people subject to domestic-violence restraining orders.

The bump-stock fight concerns the reach of a federal statute rather than the Constitution’s Second Amendment. The 1986 law bars most people from owning fully automatic machine guns or parts designed to convert weapons into machine guns. 

‘Stuck With’

The key issue is whether bump stocks meet the law’s definition of machine guns as weapons that can “automatically” discharge more than one shot “by a single function of the trigger.” Some of the court’s conservatives questioned whether that language, first used in a 1934 law that required registration of machine guns, could apply to bump stocks.

“Maybe they should have written something better,” Justice Neil Gorsuch told a Biden administration lawyer defending the ban. “One might hope they might write something better in the future. But that’s the language we’re stuck with.”

Bump stocks replace the standard stock on a rifle — the part that rests against the shooter’s shoulder — with a plastic casing that lets the weapon slide forward and backward. The device harnesses the recoil energy when a shot is fired, causing the gun to slide backward and separate from the trigger finger. The separation lets the firing mechanism reset.

By applying constant forward pressure with the non-trigger hand, the shooter can then force the rifle forward so that it “bumps” the trigger finger, even without moving the finger.

Causing ‘Carnage’

The justices spent much of the session trying to understand exactly how bump stocks work. “I’m having a little trouble with the non-trigger hand,” Chief Justice John Roberts said at one point. “Are you just holding the gun, or are you moving — pushing it forward and then back, and forward and then back?”

Roberts was one of several conservatives who didn’t give a clear indication how he will vote. Justice Clarence Thomas was also circumspect, directing his hardest questions at Jonathan Mitchell, the lawyer challenging the ban.

Thomas said the first machine gun law was enacted to address the “carnage” caused by the weapons during Prohibition.

“And behind this is the notion that the bump stock does the exact same thing,” Thomas told Mitchell. “So with that background, why shouldn’t we look at a broader definition of ‘function,’ one suggested by the government as opposed to just the narrow function that you suggest?”

Mitchell is also representing Trump in a Colorado case over the former president’s eligibility to appear on this year’s ballot despite his effort to overturn his 2020 election defeat. In the bump stock case, Mitchell’s client is Michael Cargill, a Texas man who sued after surrendering his two bump stocks, as required under the law. 

Once familiar only to gun enthusiasts, bump stocks became a national focus after the Las Vegas shooting. Five months after the shooting, Trump told the Bureau of Alcohol, Tobacco, Firearms and Explosives to come up with a way to ban the devices. 

ATF, which is part of the Justice Department, then issued a rule classifying rifles with bump stocks as machine guns, reversing a position the bureau had taken previously. At the time of the 2018 rule, ATF estimated that more than 500,000 bump stocks had been manufactured over the previous eight years. The Supreme Court let the ban take effect in 2019.

The case is Garland v. Cargill, 22-976.  

(Updates with comments from justices starting in eighth paragraph.)

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