U.S. law defines a machine gun as a weapon that automatically fires more than one shot with a single pull of a trigger. The definition also includes any parts designed to convert a weapon into a machine gun.
Federal law prohibits possessing machine guns made after 1986, with some exceptions for law enforcement, the military and certain licensed dealers. Nearly all conversion devices are illegal because they were made more recently.
People convicted of possessing machine guns and conversion devices can face up to 10 years in prison.
A Glock switch is one type of a machine gun conversion device. It's a metal or plastic piece, about the size of a coin, that attaches to the back of Glock pistol, a brand that is popular with both police and criminals. The switch interferes with a gun's internal trigger components so that it fires continuously when the trigger is pulled back and held.