A Glock switch (sometimes called a button or a giggle switch)^[1][2][3] is a small device that can be attached to the rear of the slide of a Glock handgun, changing the semi-automatic pistol into a selective fire machine pistol capable of fully automatic fire. As a type of auto sear, it functions by applying force to the trigger bar to prevent it from limiting fire to one round of ammunition per trigger pull. This device by itself, regardless if it is installed on a slide or not, is classified by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) to be a machine gun, making possession of the device illegal in the United States under most circumstances.^[4]

For reference, one model of Glock not available to civilians (the G18) has a built-in select-fire function and thus can also fire as fast as the short-recoil action can cycle. This allows the pistol to fire at approximately 1,200 rounds per minute, meaning it could empty a standard capacity 17 or extended 33 round magazine in 0.85 or 1.65 seconds respectively. The Glock 17 and 18 are identical in every way except for the selective fire function, meaning a Glock 17 can cycle at the same speed.^[5]