Acting US Attorney General Matthew Whitaker announced Tuesday that a Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) regulation has been amended to ban bump stocks, drawing a legal challenge the same day.

Whitaker said the final rule states that bump stocks fall within the federal definition of “machine gun,” so the rule clarifies existing federal regulations by adding the following language:

The term “machine gun” includes bump-stock devices, i.e., devices that allow a semiautomatic firearm to shoot more than one shot with a single pull of the trigger by harnessing the recoil energy of the semi-automatic firearm to which it is affixed so that the trigger resets and continues firing without additional physical manipulation of the trigger by the shooter.

Following Whitaker’s announcement Tuesday, a lawsuit was immediately filed to challenge the regulation.

Three gun rights groups and a bump stock owner claim that the regulation violates the Administrative Procedure Act and exceeds the ATF’s rulemaking authority. They also allege the regulation violates constitutional protections against takings without just compensation and retroactive imposition of criminal punishment as well as the constitution’s contracts clause.