House Republicans are turning up the heat on the ATF over its proposal to ban and register millions of pistol braces.

All 19 Republican members of the House Judiciary Committee signed a letter to the agency on Friday asking a series of questions about the proposal, according to a copy obtained by The Reload. The group is demanding to know how the proposal was conceived, who wrote it, and if outside agencies worked on it. The lawmakers are giving the ATF until the end of the month to answer those questions.

“ATF’s regulatory notice is deeply flawed, beyond the scope of its authority, contrary to years of previous ATF opinions, and harmful to millions of law-abiding American firearm owners,” the members said in the letter.

The letter is part of the all-out push by Republicans and the gun-rights movement to stop the ban that would affect guns equipped with shorter barrels and specialized braces in place of stocks, including popular AR-15 variants.

It comes after nearly the entire Republican Senate caucus sent a letter to the Department of Justice and ATF demanding the pistol brace ban be withdrawn. In recent weeks, Americans have flooded the Federal Register with more than 100,000 public comments opposing the wide-ranging regulation. Critics warn it would turn owners of the devices, which the Congressional Research Service has estimated number at least 10 million, into federal felons if they don’t register them with the ATF.

The letter by Republicans on the Judiciary Committee accused the Biden Administration of trying to expand ATF’s power beyond what Congress has authorized.

“Congress has not criminalized the use of a pistol arm-stabilizing brace under the GCA or allowed for its regulation under the NFA,” they said. “Through its proposed rule, ATF seeks to subject stabilizing braces to GCA criminal penalties and NFA regulation without Congressional prohibition of the underlying activity.”

The Republicans argued the standards laid out by the ATF to determine whether or not a pistol brace is legal are “arbitrary and capricious.” They also slammed the agency for contradicting previous determinations it had made on the legality of many braces. They go on to request a series of clarifications on the new standards.

The members question what a “non-operational accessory” is, the logic behind the weight measure the agency proposes, why the lack of a sight counts against a braced gun, and what “objective criteria” the ATF plans to use to measure the value of the rear surface area of a brace.

Ultimately, the Republicans said the ATF should withdraw the proposal.

“ATF’s regulation would amount to an unconstitutional infringement of fundamental Second Amendment rights,” they said. “We strongly urge ATF to abandon its proposed rule.”