The Supreme Court will hear arguments Wednesday over whether the federal government can ban devices known as “bump stocks” that allow semi-automatic rifles to act more like a machine gun.

The case doesn’t directly involve Second Amendment rights to possess firearms. Instead, it hinges on whether bump stocks meet the definition of machine guns, which are banned under a statute Congress last addressed in the 1980s.

Several bipartisan groups of lawmakers had tried to ban the devices in 2017, after authorities said a shooter had several of them on guns when he killed more than 50 people at a Las Vegas music festival. But those bills faced opposition from gun rights groups and neither chamber voted on them.

Ultimately, the Bureau of Alcohol, Tobacco, Firearms and Explosives put out a rule during the Trump administration that said bump stocks fall under the machine gun ban — so possession of them is a crime.

Now that decision faces a conservative-controlled Supreme Court with a history of skepticism for agency actions taken without a clear authority from Congress.

In court papers, both sides described bump stocks as an add-on to a rifle that uses recoil to “bump” the gun back into the trigger finger of the shooter — firing faster than usual.

Michael Cargill surrendered several of the devices and then challenged the ATF rule in Texas federal court. He argues Congress could have passed a statute with a broader definition of machine gun, or banned devices that make a semi-automatic gun fire at a rate closer to a machine gun.

“But Congress did none of these things, and neither a court nor an agency may subordinate the enacted statutory language to an actual or imagined congressional purpose,” Cargill’s brief said.

Andrew Willinger, executive director of the Center for Firearms Law at Duke University, said that if Congress had acted in the wake of the 2017 shooting, it would have short-circuited the challenge.

“Maybe we were getting to a critical point that there was enough agreement in Congress on this issue in the aftermath of the Las Vegas shooting, but it certainly doesn’t exist now,” Willinger said.

At the time, then-Speaker Paul D. Ryan, a Wisconsin Republican, called the ATF regulation “the smartest, quickest fix” to the problem.

Since then, legislation to ban bump stocks has joined the company of other gun control bills, routinely backed by Democrats and a handful of Republicans, that have not made it out of Congress.

The Democrat-controlled House advanced a bill in 2022 that included a ban of bump stocks, among other gun control measures, in response to mass shootings at an elementary school in Uvalde, Texas, and a hospital in Tulsa, Okla. But the bump stock ban was not included in a different compromise bill that Congress ultimately passed that summer.

That legislation signed into law by President Joe Biden, the first to include gun control measures in decades, focused on mental health, school safety and other measures. One of the few changes to gun laws was a temporary measure to include more records in background checks for young adults.

A Republican co-sponsor of one of the 2017 bump stock ban bills, former Florida Rep. Carlos Curbelo, said Ryan blinked when facing a revolt from conservatives in the party and gun rights groups like the National Rifle Association.

Curbelo said this month that the decision not to hold a vote on the bump stock ban bill was emblematic of a modern trend in Congress, where the majority avoids votes on bipartisan legislation for fear of backlash from their base.

“We’ll have to hope that the executive branch acts or that people bring forward legal challenges that result in lawmaking, since Congress won’t make laws,” Curbelo said. “The American people will have to hope that laws are made indirectly by the courts or temporarily by the administration.”

In this case, the U.S. Court of Appeals for the 5th Circuit found the ATF rule went beyond the definition of machine gun currently in the law. That definition rested on a single pull of the trigger, not of the trigger finger, the majority of the court wrote.

“Congress knew how to write a definition that explicitly turns on the action of a shooter rather than the action of a trigger, but chose not to do so here,” the majority wrote.

In a separate concurrence, Judge James Ho criticized the administration for trying to use a rule to expand the reach of a statute that carries criminal penalties. “Congress cannot criminalize bump stocks absent a clear and unambiguous statute,” Ho said.

A dissent from Judge Stephen A. Higginson and two others said the majority’s approach would also toss legislation used to charge people who manufacture synthetic drugs that differ slightly from currently illegal substances.

Higginson wrote that the majority’s reasoning would “legalize an instrument of mass murder.”

The Biden administration, in its appeal to the Supreme Court, argued that bump stocks fall under the definition of machine gun originally passed by Congress in 1934 in response to the Valentine’s Day massacre.

The rule banning bump stocks “addresses the special dangers posed by firearms that allow shooters to fire multiple shots without repeated manual movements,” which Congress meant to restrict from civilian use, the administration wrote.

“And a shooter need only activate the trigger once in order to fire multiple shots with a bump-stock-equipped rifle — precisely the danger that Congress sought to address,” the brief said.

Letting the decision below stand would encourage people to design devices similar to bump stocks that allow for near-automatic fire but don’t fit the rigid definition of “machine gun,” the Biden administration argued.

A group of nine senators, led by Republican Cynthia Lummis of Wyoming, urged the justices in a brief not to give the Biden administration any leeway in interpreting the statute. Giving the administration deference would make for “wild swings” in administration policy that are unfair when applied to criminal law.

“It is unsettling to think that something that is legal one day can be subject to criminal sanctions the next with no intervening act of Congress,” the brief said.

Although the justices expanded Second Amendment rights in a 2022 decision, New York State Rifle & Pistol Association Inc. v. Bruen, Willinger said Congress may still be able to address bump stocks.

“There’s not much question that Congress could have obviated all this and solved the issue for any of this litigation,” Willinger said.

Willinger pointed out that no appeals courts since the Bruen decision have ruled against the machine gun ban in the National Firearms Act.