The federal government has only just begun the process of banning bump stocks for firearms, but Second Amendment groups are already lining up to challenge whatever the Justice Department produces.

Americans have a right to the aftermarket gun accessory, the groups insist, and the Obama administration repeatedly said it didn’t have the power to regulate bump stocks en masse without changes to federal law.

That could undercut the Trump administration’s attempt to write a ban without authority from Congress.

“I think you’re definitely going to see a challenge — perhaps multiple challenges,” said Brandon Combs, president of the Firearms Policy Coalition.

The coalition said it has already retained attorneys to prepare for the legal fight.

Gun owners in Florida have laid the groundwork by suing to stop a portion of a new state law that bans sales or even possession of bump stocks within its borders.

One lawsuit says that amounts to a seizure of property without compensation — a violation of the Fifth Amendment, says the complaint filed by five Floridians who own bump stocks or similar devices.

They cited the Obama administration’s rulings, including a 2010 decision by the Bureau of Alcohol, Tobacco, Firearms and Explosives that found one particular bump stock was a “firearm part” and not subject to weapons regulations.

Bump stocks are add-ons used by gun owners to increase the rate of fire of semi-automatic rifles, making them approximate the rate of fire of machine guns. Machine guns are heavily restricted, and semi-automatic rifles are not.

Their use has been deeply controversial since the Las Vegas massacre last year. Police said the gunman used a number of bump stocks attached to rifles to spray bullets from his hotel room down onto a crowd at a music festival.

He fired about 900 rounds in a 10-minute span, leaving 58 people dead and more than 800 injured — about half of those from gunshot wounds.

In the wake of that shooting and the massacre last month at a Florida high school — where bump stocks were not used — President Trump ordered the Justice Department to pursue a rules change that would ban sales of the devices.

Attorney General Jeff Sessions started the rule-making process Friday by opening a 90-day comment period.

Erich Pratt, executive director of Gun Owners of America, said his group plans to seek out other pro-gun organizations to join a court challenge. He said attorneys are mapping out a strategy.

“We predict that an honest, constitutional and legal analysis by the courts will result in this regulation being struck down,” Mr. Pratt said.

Significant legal questions have been raised over whether the government can regulate bump stocks under federal statutes.

Under President George W. Bush, the ATF ruled that one particular device, the Akins Accelerator, was outside the boundaries of federal regulations.

But the bureau reversed itself in 2006 by issuing a directive saying that a device designed to convert a weapon to fire multiple rounds automatically is a “machine gun” under federal law.

Federal law defines a machine gun 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.”

Regular citizens are generally banned from owning machine guns manufactured after 1986.

The Obama administration subsequently determined on multiple occasions that it couldn’t issue a blanket ban on all the devices. It said certain models didn’t provide “automatic action” and required multiple trigger pulls to fire multiple bullets.

Mr. Sessions, though, said his department has reviewed these decisions, concluded that they were wrong and that ATF does have the power to impose regulations.

“ATF’s classifications of bump-stock devices between 2008 and 2017 did not include extensive legal analysis of these terms in concluding that the bump-stock-type devices at issue were not ‘machineguns,’ ” Mr. Sessions said.

In its proposal, the department said the rulings needlessly focused on certain devices’ lack of mechanical parts, such as springs, to engage automatic firing when some of them simply leveraged the recoil of the gun to fire multiple rounds with a single “function” of the trigger.

Asked about potential legal challenges to the new regulations, a Justice Department spokeswoman declined to comment Tuesday, citing the ongoing rule-making process.

Still, Thomas E. Brandon, the acting ATF director, acknowledged difficulties in delivering a new interpretation. He said the more concrete way to ban bump stocks would be legislation from Congress either imposing the ban or making it clear that ATF has the authority to do so.

“A law is clearly the best route, but we’re doing everything within the executive branch that we can do, and that’s through the administrative procedures act, to enhance public safety, to keep an open mind about bump stocks,” he said at a Senate Judiciary Committee hearing.

Gun control groups argue that the Justice Department does have the power to regulate the devices but are calling on the ATF to clean up its conflicting rulings.

“Until ATF finalizes a rule outlining specifically what parts and functions fall within the statutory definition, manufacturers will continue to exploit this vague and insufficient administrative law,” legal representatives for the group Everytown for Gun Safety wrote earlier this year.