A bump stock is an attachment that can be added to a semi-automatic rifle to assist in bump firing, which is the act of using the recoil of a firearm to fire ammunition cartridges in rapid succession. Essentially, bump stocks assist rapid fire by "bumping" the trigger against ones finger, allowing the firearms recoil, plus constant forward pressure by the non-shooting arm, to actuate the trigger. The stock replaces a rifle’s standard stock, which is the part held against the shoulder, and frees the weapon to slide back and forth rapidly, harnessing the energy from the kickback shooters feel when the weapon fires. Bump stocks do not make the firearm automatic, but they allow a weapon to fire at nearly the rate of a machine gun without technically converting it to a fully automatic firearm.

Bump stocks were the subject of controversy after the 2017 Las Vegas shooting, where the shooter used bump stocks to fire on a crowd of concertgoers, killing 60 people and injuring hundreds more. In response, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) issued a rule in 2018 that expanded the definition of "machine gun" prohibited under the National Firearms Act to include bump stocks. The rule went into effect on March 26, 2019, and current possessors of bump stocks were required to divest themselves of possession as of that date. Possessors had the option to destroy the device or abandon it at the nearest ATF office.

The legality of the bump stock ban has been challenged in court, and the Supreme Court has agreed to weigh in on the matter. The case centers on whether bump stocks fall into the definition of a machine gun, and whether the ATF had the authority to ban them under the National Firearms Act.