Trigger (firearms)
![Trigger mechanism in a bolt-action rifle.](/Images/godic/202501/09/Trigger_mechanism_bf_19232422.jpg")
![Thompson submachine gun trigger](/Images/godic/202501/09/Thompson_submachine_gun_Firecontrols2422.jpg")
![The double-crescent trigger on the MG 34, which enabled select fire capability without using a selector switch. Pressing the upper segment of the trigger produced semi-automatic fire, while holding the lower segment of the trigger produced fully automatic fire.](/Images/godic/202501/09/MG_34_trigger_RCR_Museum2422.JPG")
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.