single action trigger

A trigger is a mechanism that actuates the firing sequence of a firearm, airgun, crossbow, or speargun. A trigger may also start other non-shooting mechanisms such as a trap, a switch 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 or the Iron Horse TOR (thumb operated receiver) use a thumb-actuated design, and others like the Springfield Armory M6 Scout use a "squeeze-bar trigger" similar to the ticklers on medieval European crossbows.

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