A single-action firearm is a type of firearm mechanism where the trigger performs a single function: releasing the hammer or striker to strike the firing pin, which in turn ignites the primer and fires the cartridge. With single-action revolvers, the shooter must manually cock the hammer before each shot, setting it in the “cocked” position. This design provides a light and consistent trigger pull, making these firearms known for their accuracy, especially in target shooting or western-style revolvers.

Single Action Only pistols (SAO) require the operator to manually load the first round into the chamber by operating the slide and thus “cocking” the hammer or striker. One or multiple safety mechanisms are typically present on an SAO firearm to prevent an accidental discharge. Once the first round is fired, the semiauto mechanism will re-cock the hammer or striker with each shot until the magazine is empty.