TRIGGER ACTION TYPES
Trigger actions refer to how a firearm operates with each trigger press. There are two basic types of trigger actions: single-action and double-action. Some firearms incorporate both.
The trigger action releases the striker or hammer, so it causes the primer on a cartridge to be struck and fire a shot. Depending on the action type, other operations (to prepare the firearm to fire) may be tied to each trigger press.
A striker-fired gun uses a striker rod or firing pin driven by a spring that hits the bullet primer. Striker-fired handguns are very popular and straightforward to operate. These designs incorporate the safety mechanism into the trigger, and all the firing mechanisms are internal to the firearm.
A hammer-fired design uses a hinged lever that strikes a firing pin that hits the bullet primer. Hammer-fired designs are used in 1911 designs, revolvers, and many competition pistols.
The single-action trigger performs one action for each trigger pull. That signal action releases the striker or hammer, so it hits the bullet primer, and the gun fires. Because single-action is just a simple release operation, it takes another action (recoiling of the slide on the pistol) to cock the hammer/striker for the next shot. The single-action is usually a lighter trigger pull because it is only performing one action to fire the pistol.
Double-Action (DA)/Double-Action Only (DAO)
The double-action trigger performs two actions for each trigger pull. Each trigger press will cock the hammer or firing pin, then release them. Revolvers provide a good example of a double-action-only design because you can see both actions at work as you pull the trigger. When you start to pull the trigger, the cylinder will rotate by moving the cartridge into place. Then the hammer is pulled back in preparation to fire. As you continue through the press, the cylinder stops, and the hammer is released. A double-action trigger can feel heavier, as it is performing two operations in order to fire the pistol.
A double-action striker-fired system can feel more like a single-action trigger but as you pull the trigger it does perform an internal cocking and release of the internal striker.
Double-action and single-action pistols start in double-action for the first trigger pull, then they perform in single-action thereafter. This means the first trigger press will cock the hammer or striker and fire a shot. The recoil from the first shot will cause the pistol to cycle. The cycling is used to load another cartridge and cock the hammer or striker. The subsequent trigger pulls will be single action, because the cycling with each recoil is used to prep the pistol for the next shot.