A legal cloud descended over many American gun owners on Monday.

A new rule proposed by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) at the request of President Joe Biden would make most firearms with stabilizing pistol braces illegal. Owners would have to register, turn in, or disassemble the guns to avoid federal felony charges. One government estimate found as many as 40 million guns could be affected.

“It will be the largest gun registration, destruction, and confiscation scheme in American history,” Alex Bosco, who invented the stabilizing brace and founded the biggest manufacturer of them, told The Reload.

The rule outlines a complex point system the agency wants to create to judge when a gun with a stabilizing brace should be classified as a pistol and when it should be classified as a short-barrel rifle requiring registration under the National Firearms Act of 1934 (NFA). The proposal aims to reconcile the ATF’s decade-long track record of approving the braces on a case-by-case basis. That process was often criticized by members of the gun industry and gun-rights movement as arbitrary and contradictory.

But gun advocates said the new system is worse than the old. While the system gives objective measures for the weight or length of the guns affected, opponents pointed to the seemingly random criteria used to determine which braces are legal and which aren’t. They said factors like whether the gun can be easily fired with one hand or whether an attached brace is adjustable in length have no basis in the statute, which sets the line between pistols and short-barreled rifles at whether the gun is “designed and intended” to be pressed against the shooter’s shoulder.

“Today’s proposed rulemaking on pistol-braced firearms represents a gross abuse of executive authority,” said Aidan Johnston, Director of Federal Affairs for Gun Owners of America, in a statement.

Bosco said the rule would outlaw the vast majority of braces on the market and read like it was “reverse-engineered to make braces illegal.” He called it “arbitrary and capricious.”

April Langwell, chief of the ATF’s Public Affairs Division, told The Reload the one-handed-firing standard comes from pre-existing definitions of handgun and pistol from federal statute. However, the ATF did not provide further details.

“The Notice of Proposed Rulemaking on stabilizing braces has been published for public notice and comment pursuant to the Administrative Procedure Act,” she said. “The NPRM sets forth in detail the proposed rule, including its legal and factual basis. While the statutory notice and comment period is ongoing, ATF and the Department of Justice cannot further comment on the specific content of the proposed rule.”

In the proposal, the ATF said the rule was necessary to prevent people from using pistol braces as a way of circumventing the NFA’s registration requirements. They said it was important to keep Americans from owning the guns in question because they are “especially dangerous and unusual.”

However, the ATF only lists two crimes committed with pistol braces—the mass shootings in Boulder, Colorado and Dayton, Ohio—in the proposal. Instead, it cites Congress’s judgment in passing the NFA as the reason it is necessary to close what they view as a loophole in the law.

“Congress placed stricter requirements on the making and possession of ‘short-barreled rifles’ because it found them to pose a significant crime problem,” the agency wrote in the proposal. “Providing clarity to the public and industry on how ATF enforces the provisions of the [National Firearms Act of 1934] through this proposed rule significantly enhances public safety and could reduce the criminal use of such firearms, which are easily concealable from the public and first responders.”

The rule is the result of executive action taken by President Biden. Biden has pressed for an aggressive expansion of gun-control laws since taking office but has made no progress on his legislative agenda on guns because the measures he wants can’t garner 60, or sometimes even 50, votes in the divided Senate. Instead, as part of his effort to bypass Congress and unilaterally impose new gun restrictions, he directed the Department of Justice and ATF to create proposals to ban so-called ghost guns and certain pistol braces in the aftermath of mass shootings in Georgia and Colorado.

A far-reaching gun registration and confiscation effort could further degrade Biden’s already poor polling on gun policy. It could also hamper his party’s attempt to hold control of Congress in an election slated to take place after more than 8 million Americans bought a gun for the first time last year, and as gun buying continues at a record pace.

In the proposal, the ATF estimates there are between 3 and 7 million braces already owned by Americans, and about 1.9 million would be affected by the rule. It estimated altering or destroying the banned braces would cost Americans more than $440 million. But the ATF’s estimates are far lower than others.

Bosco estimated his company, SB Tactical, made at least 2 million of the best-selling SBA3. The ATF’s proposal specifically notes the point system outlaws that model. SB Tactical makes several more models beyond the SBA3, and there are at least seven other companies that also make braces.

The Congressional Research Service estimated civilians already owned 10 to 40 million braces in April 2021.

“Altering the classification of firearms equipped with stabilizing braces would likely affect millions of owners,” the nonpartisan agency concluded.

Gun-rights proponents said they are planning to fight the proposal. Once the ATF officially enters the proposal into the federal register, the public will have 90 days to submit comments for or against it. President Donald Trump withdrew a similar proposal on stabilizing braces submitted last year after receiving opposition during the comment period.

However, a similar ban and confiscation effort targeting bump stocks was carried out through rulemaking by Trump’s administration despite an overwhelmingly negative comment period. That rule has faced legal trouble in recent months and was struck down in the Sixth Circuit Court of Appeals.

The Firearms Policy Coalition is already considering legal action should the brace rule go into effect.

“Just as FPC did when the Trump Administration’s DOJ lawlessly declared that bump-stocks were machine guns, we will use every available resource to defend the People’s human right to keep and bear arms from attacks by the ATF,” Adam Kraut, the group’s senior director of legal operations, said in a statement. “We stand ready to defend the People, human liberty, and personal property in this rulemaking process and, if necessary, in court.”

Update 3:31 pm: This story has been updated to include comment from the ATF.