Trigger (firearms)
A trigger is a mechanism that actuates the firing sequence of a firearm. Triggers almost universally consist of levers or buttons actuated by the index finger. Some variations use the thumb or weak fingers to actuate the trigger, a feature typically found on mounted weapons. Examples are the M2 Browning machine gun and the Springfield Armory M6 Scout.
Read more about Trigger (firearms): Function, Actions (mechanisms)