A trigger is a mechanism that actuates the firing sequence of a firearm or crossbow; a trigger may also start another mechanism such as a trap or a quick release. A small amount of energy applied to the trigger causes the release of much more energy. Triggers usually consist of levers or buttons actuated by the index finger. Some variations use the thumb or weak fingers to actuate the trigger, a feature found on mounted weapons such as the M2 Browning machine gun and the Springfield Armory M6 Scout.