The Supreme Court wrestled on Wednesday over whether the Trump administration acted lawfully in enacting a ban on bump stocks after one of the deadliest mass shootings in U.S. history.

The justices appeared split largely along ideological lines over the ban, which prohibits the sale and possession of bump stocks, attachments that enable semiautomatic rifles to fire at speeds rivaling machine guns. Some raised concerns about the broader implications of a reversal.

The case does not turn on the Second Amendment right to keep and bear arms. Instead it is one of a number of challenges aimed at curtailing the power of administrative agencies — in this instance, the Bureau of Alcohol, Tobacco, Firearms and Explosives. A decision is expected by late June.

After a gunman stationed on the 32nd floor of a hotel suite opened fire at a country music festival in Las Vegas in 2017, a ban on bump stocks gained political traction, one of the few pieces of gun control legislation to generate significant discussion. Officials at the Justice Department initially said the executive branch could not prohibit the accessory without action by Congress. But it ultimately reversed course and enacted a ban on its own.

At issue is whether a bump stock falls within the legal definition of a machine gun. If the court deems a bump stock can be used to make a gun into a “machine gun,” then it can be prohibited as part of a category heavily regulated by the A.T.F.

During less than two hours of arguments, the justices appeared to struggle to make sense of the mechanics of gun triggers and the value of a ban for gun owners and the wider public.

“Look, intuitively, I am entirely sympathetic to your argument,” Justice Amy Coney Barrett said to the lawyer representing the government, Brian H. Fletcher. “I mean, it seems like, yes, this is functioning like a machine gun would.”

But she also wondered aloud why Congress had not enacted legislation that more explicitly covered such devices.

Justice Neil M. Gorsuch, one of the court’s most conservative members, appeared to agree. He said that while he could “certainly understand why these items should be made illegal,” he failed to see why an administrative agency, rather than Congress, should be the one to act.

The remaining conservatives on the court appeared splintered. Chief Justice John G. Roberts Jr., who is often near the court’s center and a potentially crucial vote in the case, asked few questions, giving little insight into his position.

The three liberal justices seemed to be unified in their support for the government’s decision to ban the device, repeatedly raising hypotheticals about the bump stock and how it enabled semiautomatic guns to fire a “torrent of bullets.”

The Justice Department’s abrupt shift in tack seemed to trouble Justice Brett M. Kavanaugh.

If the device were “a new thing, obviously, covered by this old statutory language, you would expect the Bush administration and the Obama administration and Senator Feinstein to say, of course, it’s covered, and they didn’t,” Justice Kavanaugh said, singling out Senator Dianne Feinstein, who died in September and was one of the most notable critics of bump stocks. “And that’s reason for pause.”

The challenge was brought by Michael Cargill, a gun shop owner in Texas who sold bump stocks. He is represented by the New Civil Liberties Alliance, a legal advocacy group with financial ties to Charles Koch, a billionaire who has long supported conservative and libertarian causes. The group primarily targets what it considers unlawful uses of administrative power.

Jonathan F. Mitchell, the lawyer arguing on behalf of Mr. Cargill to overturn the bump stock ban, posited that there could be “a valid reason” for allowing bump stocks because they can help people with disabilities or arthritis to shoot.

Justice Sonia Sotomayor quickly retorted, “Why would even a person with arthritis, why would Congress think they needed to shoot 400 to 7- or 800 rounds of ammunition under any circumstance?”

Some of the conservative justices, notably Justices Samuel A. Alito Jr., Gorsuch and Kavanaugh, raised concerns about the effect of the ban on people who had bought bump stocks before the accessories were prohibited.

Under the federal ban, possession or sale of bump stocks could result in prison time. Those who sold or possessed bump stocks at the time of the ban were asked to turn them in or destroy them.

Justice Alito asked Mr. Fletcher what would happen to people who have owned bump stocks in the years since the ban, including those living in the part of the country where a federal appeals court overturned the ban.

“Can they be prosecuted?” Justice Alito asked.

When Mr. Fletcher answered “probably yes,” Justice Alito quickly responded, “Isn’t that disturbing?”

“That’s going to ensnare a lot of people who are not aware of the legal prohibition,” Justice Kavanaugh said.

Mr. Fletcher’s response, that the agency had completed the requisite steps of notifying the public about the ban, elicited a sarcastic response from Justice Gorsuch.

Most Americans were surely spending their leisure time perusing the Federal Register, the publication where the government notes proposed rules, the justice said. He mockingly added that “gun owners across the country” were certain to “crack it open next to the fire and the dog.”

Mr. Fletcher wryly noted that the ban had “not gone unnoticed” by gun enthusiasts, citing the many legal challenges that followed.

Machine guns were banned under the National Firearms Act of 1934, which defines the firearm 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.” The definition was broadened by the Gun Control Act of 1968 to include parts that can be used to convert a weapon into a machine gun.

But until the Trump administration’s ban, bump stocks were an exception, considered legal on the grounds that they increased the speed of a gun by sliding the stock back and forth to rapidly pull the trigger, not by “a single function of the trigger” as required for a machine gun.

The justices did not appear to share a common understanding of the bump stock and precisely how it worked. Several appeared to be puzzling through the mechanics. Justice Kagan motioned with her arms and hands as she sought to demonstrate her understanding of how the device transformed a weapon.

Neither did they have a unified interpretation of whether the bump stock changed the function of a trigger on semiautomatic weapons, which are devised to fire a round with each pull of the trigger.

The technical parts of the argument focused on the wording of the law, specifically key terms like “automatically” and “a single function of the trigger.”

Mr. Mitchell argued that the justices should take a narrow reading of the law, meaning it would not apply to bump stocks.

Justices Ketanji Brown Jackson and Alito both engaged in extended back-and-forths with the lawyers, pressing them on how to interpret the statute.

Justice Alito appeared supportive of Mr. Mitchell’s argument that the ban overreached because the device itself does not physically touch and move the trigger mechanism in a rifle.

Justice Jackson seemed skeptical of that argument. She appeared more supportive of the government’s argument that the device could be banned because of its practical effect on a gun, sharply increasing the speed and number of bullets it can fire.

Justice Kagan appeared to agree.

“I mean, maybe you could use the device differently, but the entire point of this device is that you exert forward pressure and you have your finger on the trigger, and then a torrent of bullets shoots out,” she said.

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