Bump stocks are accessories that can be added to semiautomatic rifles to increase the rate that they fire. Bump stocks replace the standard stock, the part of the gun that rests against the holder’s shoulder. They trigger the gun to continuously fire as long as the shooter holds down the trigger and applies pressure to the stock. They do not convert a semiautomatic rifle into automatic rifles, but they do allow semiautomatic rifles to shoot nearly as fast. They’re also known as “reciprocating stocks” and “multiburst trigger activators” (the legal term in California law).