The WILL’s lawsuit, filed on behalf of military veterans in Wisconsin and Texas, notes that the rule could apply to as many as 40 million people who would be required to pay a tax on the registered weapon and could face a fine or imprisonment for failure to comply.

The suit argues the braces — which were invented to help disabled veterans fire their pistols safely — make many types of pistols more accurate and therefore safer. It adds that the veterans use the braces either because they have life-altering injuries suffered during military service, or for firearms instruction and training purposes.

Their lawsuit alleges that ATF’s new rule violates the Second Amendment and the Separation of Powers, which prohibits federal agencies from making new laws without clear Congressional authorization.

“These military veterans defended our country overseas, and now they are defending our rights here at home. WILL is proud to represent these patriots. The Biden Administration has no power to re-classify pistols as rifles, and we will vigorously defend the Second Amendment in federal court,” said WILL Deputy Counsel, Dan Lennington.

FPC’s suit alleges the actions of the Justice Department and ATF violate both the Administrative Procedure Act and the Second Amendment to the United States Constitution.

It seeks an injunction against the so-called “final rule” being enforced as well as clarification on its legality.

“Federal agencies do not have the power to write new laws, and yet the ATF continues to attempt to expand its authority using the federal rulemaking process,” said Cody Wisniewski, FPC’s Senior Attorney for Constitutional Litigation.

“This ‘rule’ is, in effect, a federal law that will transform millions of peaceable people into felons overnight simply for owning a firearm that has been lawful to own for decades. We won’t stand idly by while the ATF tramples the rights of millions of peaceable individuals.”

Announcing the change last month, Attorney General Merrick Garland, said the rule was in line with his department’s priority to keep communities safe from gun violence.

“Almost a century ago, Congress determined that short-barreled rifles must be subject to heightened requirements. Today’s rule makes clear that firearm manufacturers, dealers, and individuals cannot evade these important public safety protections simply by adding accessories to pistols that transform them into short-barreled rifles,” Garland said.

ATF Director Steven Dettelbach said the reason short-barreled rifles and sawed-off shotguns should be subjected to greater legal requirements than most other guns was that short-barreled rifles have the greater capability of long guns, but were easier to conceal, like a pistol.

Individual gun owners, manufacturers and dealers have a 120-day grace period from Jan. 13, the day the rule was published in the Federal Register, to register tax-free any existing National Firearms Act short-barreled rifles covered by the rule.

Other options include removing the stabilizing brace to return the firearm to a pistol or surrendering covered short-barreled rifles to ATF.