Single-action

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.

In "double action" firearm designs, the trigger is also used to cock the firearm - and there are many designs where the trigger is used for a range of other functions. Although triggers usually consist of a lever actuated by the index finger, some such as the M2 Browning machine gun use the thumb, and others like the Springfield Armory M6 Scout use a "squeeze-bar trigger."

Firearms use triggers to initiate the firing of a cartridge in the firing chamber of the weapon. This is accomplished by actuating a striking device through a combination of spring and kinetic energy operating through a firing pin to strike and ignite the primer. There are two primary types of striking mechanisms, hammers and strikers. Hammers are spring-tensioned masses of metal that pivot on a pin when released and strike a firing pin to discharge a cartridge. Strikers are, essentially, spring-loaded firing pins that travel on an axis in-line with the cartridge eliminating the need for a separate hammer. The connection between the trigger and the hammer is generally referred to as the sear surface. Variable mechanisms will have this surface directly on the trigger and hammer or have separate sears or other connecting parts.

There are numerous types of action, where action refers to the mechanism (the trigger, hammer, and safeties considered as a unit) or to the logic of how it is built and how it is used. They are categorized according to which functions the trigger is to perform. In addition to releasing the hammer or the striker, a trigger may cock the hammer or striker, rotate a revolver's cylinder, deactivate passive safeties, select between semi-automatic and full-automatic fire such as the Steyr AUG (see progressive trigger), or pre-set a "set trigger". Most modern firearms use the trigger to deactivate passive safeties but this does not change how they are identified.

...
Wikipedia