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WASHINGTON — A federal appeals court dealt a legal setback to the Biden administration on guns Tuesday in a lawsuit challenging tighter regulations on stabilizing braces, an accessory that has been used in several mass shootings in the U.S.

The Fifth Circuit Court of Appeals temporarily blocked the Biden administration’s rule from going into effect for the gun owners and groups who filed the lawsuit. The order came shortly before a deadline that would have required people to register stabilizing braces and pay a fee, or remove the braces from their weapons.

The Bureau of Alcohol, Tobacco, Firearms and Explosives finalized the rule earlier this year after finding the accessories make pistols as dangerously powerful and easy to conceal as short-barreled rifles or sawed-off shotguns, weapons that have been highly regulated since the days of Al Capone.

The rule was quickly challenged by gun-rights groups that argued it violated Second Amendment protections by requiring millions of people to alter or register their weapons. They argued the ATF found a decade ago that the braces did not make pistols like short-barreled rifles.

The accessories, also known as pistol braces, let a shooter fire one handed by attaching to the back of a gun, lengthening the weapon and strapping to the arm. The were originally developed for disabled veterans, though gun-control groups say they became a loophole exploited by gunmakers to make weapons more deadly.

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 full impact of the decision wasn’t immediately clear. The order applied only to the plaintiffs in the case: two gun owners, a company that makes pistols with stabilizing braces, and a gun-rights group. The appeals court did not say whether the rule was blocked for others, including people who buy the guns from the company, Maxim Defense Industries, and members of the Firearms Policy Coalition.

An attorney on the case, Erik Jaffe, said they were pleased with the ruling and looking forward to arguments in the case, which the Fifth Circuit also ordered be argued quickly.

The lawsuit came before the court after the Firearms Policy Coalition appealed a lower-court order from a Texas judge who declined to block enforcement of the rule as the case played out.

President Joe Biden first announced the regulation 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. In March, a shooter who killed three students and three staff members at a Christian school in Nashville also used one.

Attorney General Merrick Garland said when the rule was finalized that it would help keep communities safe from gun violence.

The ruling comes amid upheaval in the legal landscape for guns after the U.S. Supreme Court’s conservative majority set new standards for reviewing the nation’s firearm laws, requiring that they be “consistent with the Nation’s historical tradition of firearm regulation.”

The Fifth Circuit also ruled in February that the government can’t stop people who have domestic violence restraining orders against them from owning guns.