HELENA (KPAX) — Montana Attorney General Austin Knudsen has led 21 other attorneys general from across the country, filing a brief in support of a lawsuit challenging a federal rule banning bump stocks.

Plaintiffs, including the Gun Owners of America, have asked the U.S. Supreme Court to rule that the Bureau of Alcohol, Tobacco, Firearms and Explosives, or ATF, went against federal law and proper procedure when it made a rule that required gun owners to give up or destroy their bump stocks.

Knudsen’s amicus brief says the ATF rule would violate the right to keep and bear arms “by immediately transforming hundreds of thousands of law-abiding gun owners in the States into criminals.”

A bump stock redirects the recoil energy when a semi-automatic rifle is fired, moving the gun back and forth against the trigger finger and allowing someone to fire more quickly – similar, but not identical, to a fully automatic weapon.

In 2018, following a mass shooting in Las Vegas, President Donald Trump issued a memo, asking his attorney general to propose a rule “banning all devices that turn legal weapons into machineguns.” Later that year, the ATF announced a rule clarifying that bump stocks fall within the definition of machine guns, already restricted under federal law.

Knudsen’s brief argues the agency’s interpretation of the existing federal law was essentially rewriting the statute without going through Congress.

“When the ATF—or any agency—invades protected rights by interpreting statutes too broadly, this Court should step in,” he said.

Attorneys general from West Virginia and 20 other states – all Republicans – joined Knudsen’s brief.

An appeals court panel initially ruled against the ATF, but the full court split on the issue – leaving the ban in place for now. The Supreme Court has not yet determined whether it will take up the case.