WASHINGTON — A conservative law firm challenged new federal regulations on guns with stabilizing braces Tuesday, suing to block a gun-control action touted by President Biden after the accessories were used in two mass shootings.

Attorney General Merrick Garland, shown on Jan. 12, said stabilizing braces transform a handgun into a weapon that’s powerful and easy to conceal. Manuel Balce Ceneta/Associated Press

The lawsuit, filed in federal court in Texas, challenges the move to treat the guns like short-barreled rifles, a weapon like a sawed-off shotgun that has been heavily regulated since the 1930s.

The case against the Bureau of Alcohol, Tobacco, Firearms and Explosives argues that millions of people have guns with the braces and use them to make firing “more accurate, and therefore safer.”

The lawsuit was filed on behalf of three veterans by the Milwaukee-based Wisconsin Institute for Law and Liberty and comes as the new federal rule takes effect Tuesday. They argue in the lawsuit that the new ATF rule forces owners into “unthinkable choices” of removing the brace, submitting to a national registry or opening themselves up to possible charges.

“The new rule unlawfully usurps Congressional authority by significantly expanding the definition of ‘rifle’ under federal law and, with it, imposes potential criminal liability on millions of Americans exercising their Second Amendment rights,” the lawsuit argues. A decade ago, the ATF found that the braces did not make guns similar to short-barreled rifles.

At least three million guns with stabilizing braces are in circulation in the U.S., according to the ATF. Other estimates place the number much higher, the suit claims. The plaintiffs are asking the court to block enforcement of the rule.

The regulation was one of several steps Biden announced in 2021 after a man using a stabilizing brace killed 10 people at a grocery store in Boulder, Colorado. A stabilizing brace was also used in a shooting in Dayton, Ohio, that left nine people dead in 2019.

The agency declined to comment on the lawsuit, citing the pending litigation. The case is believed to be the first to challenge the rule that’s drawn pushback from gun rights groups.

Stabilizing braces transform a handgun into a weapon that’s powerful and easy to conceal, Attorney General Merrick Garland said when he announced the rule earlier this month. Originally developed for disabled veterans, the accessories became a loophole exploited by gunmakers to make weapons more deadly, gun-control groups said.