Auto sear
An auto sear ("automatic sear") is a part of an automatic firearm that holds the hammer in the cocked position while the bolt of the weapon is cycling. An auto sear is required in nearly every automatic rifle, especially assault rifles. An auto sear allows a semi-automatic gun to be converted into one capable of automatic firing with a single, continuous pull of the trigger. The Bureau of Alcohol, Tobacco, Firearms and Explosives designated any firearm capable of firing more than one round with a single operation of the trigger mechanism as a "machine gun", and the possession of such devices is illegal under United States federal law if the person in possession does not have the required licensing. See also 

 Glock switch

References 

Firearm components