A bump-fire stock (or bump stock) is an aftermarket product designed to make a semi-automatic rifle fire faster. It is used to replace the rifle’s stock — the part held against the shoulder — freeing it to slide back and forth rapidly and harness the recoil energy.
Bump stocks came under scrutiny due to the 2017 mass shooting in Las Vegas as a dozen of the rifles used by the shooter were modified with bump stocks, allowing the shooter to reportedly fire more than 1,100 rounds in 11 minutes. However, bump-fire stocks have been available for many years in the U.S.