When former President Donald Trump banned the gun components known as “bump stocks” in 2018, following a horrific mass shooting at the Route 91 Harvest Music Festival in Las Vegas, it set off a legal firestorm. On Wednesday, battle reached the Supreme Court, where conservative justices appeared to be skeptical in oral arguments that Trump had the authority to use an agency rule to enact the ban.

Bump stocks harness the recoil energy of a gun to slide back and forth, allowing a shooter to fire hundreds of rounds per minute and enabling a semiautomatic weapon to fire at rates similar to an automatic one.

It was far from clear how the high court would rule after Wednesday’s arguments in the case, Garland v. Cargill. But conservative justices repeatedly pressed deputy solicitor general Brian Fletcher about why Congress had not specifically banned bump stocks, and expressed concern about what would happen to people who’d bought them lawfully.

“The question is, why didn’t Congress pass that definition to cover this more clearly?” Justice Amy Coney Barrett asked.

Congress banned machine guns in 1934 with the National Firearms Act. That legislation generally criminalizes civilian possession of weapons that fire by a “single function of the trigger.” The 1968 Gun Control Act included modification devices as part of the definition for machine guns.

The Bureau of Alcohol, Tobacco, Firearms and Explosives first classified bump stocks as legal in 2006. After the Las Vegas shooter used them to fire more than 1,000 rounds in the deadliest shooting by a single gunman in U.S. history, the Trump administration directed the ATF to reassess them.

In 2018, the agency’s new rule classified bump stocks as modifications that create machine guns, making them illegal under federal law. Austin gun store owner and firearms instructor Michael Cargill surrendered his two bump stocks that year and then sued to overturn the rule, arguing that only Congress had the authority to change the law, and that bump stocks do not meet the definition of “machine gun” because they require the shooter to hit the trigger each time the gun is fired.

But the government’s lawyers argued otherwise Wednesday. “Those weapons do exactly what Congress meant to prohibit,” Fletcher said. “Those weapons are machine guns.”

Most of the debate consisted of an often tedious discussion over the grammar over the 1934 and 1968 legislation.

Because “function” is an intransitive verb, the court and the opposing lawyers all seemed to agree, that must mean that it refers to the weapon itself; a shooter does not “function” something, grammatically speaking

But Jonathan Mitchell, who represented Cargill, contended that for the “function of the trigger” to amount to machine gun fire, it had to continue firing automatically after a single press.

“Only a trigger can have a function, not a shooter,” Mitchell told the court. “If the statute had actually said a single pull of the trigger, then that would refer to the shooter, because a trigger certainly can’t pull itself,” he said.

Fletcher, by contrast, argued that the “function” referred to the overall effect of working the trigger, including setting off a series of rapid fire shots with one pull. The “natural meaning” of the phrase, Fletcher argued, referred to the shooter’s action.

That definition appeared to resonate with Justice Ketanji Brown Jackson.

“It can function in the same way [as a machine gun] in so far as it automatically allows 800 rounds to be released,” Jackson said.

The justices delved deeply into the firing mechanism and whether the language Congress used to ban machine guns seven decades before bump stocks existed could be construed to include them.

The Court’s three liberal justices appeared skeptical that a plain reading of the law wouldn’t include include modification devices like bump stocks, which allow shooters to reach machine gun rates of fire by initiating a single shooting sequence.

“There’s a discrete human action that produces a torrent of bullets, and that’s exactly what’s happening here,” Justice Elena Kagan said, adding: “At some point you have to apply a little bit of common sense to the way you read a statute.”

But the case may hinge on the conservative justices’ interpretation of whether the reclassification of bump stocks should have required Congress to step in.

Justice Samuel Alito called the prospect of prosecuting people who have possessed the formerly legal devices “disturbing.” Justice Brett Kavanaugh said the fact that Congress hadn’t banned bump stocks while both Democratic and Republican administrations classified them as legal was “reason for a pause.”

And Justice Neil Gorsuch worried that an interpretive rule from the ATF could make felons out of hundreds of thousands of people who bought the weapons when they were still legal.

Gorsuch also questioned how the ATF could flip its position when the George W. Bush administration and the Obama administration both viewed them as legal. The late California Sen. Dianne Feinstein, a Democrat and strong advocate for gun control, had also decried the agency ban as an easily contestable measure that would undermine congressional attempts to ban bump stocks under federal law.

“It is a very old statute, and it was designed for an obvious problem in the 1930s, Al Capone,” Gorsuch said of the National Firearms Act. “And that’s what they wrote. Maybe they should have written something better. One might hope they write something better in the future.”

Sara Boboltz contributed reporting.