An action is the mechanical part of a gun that performs the cycle of operation.

related:

• action, autoloading
• action, automatic
• action, Ballard
• action, barreled
• action, blow-forward
• action, blowback
• action, bolt
• action, boxlock
• action, break-open
• action, delayed blowback
• action, dropping block
• action, falling block
• action, hinged frame
• action, lever
• action, locked breech
• action, open breech
• action, pivoting block
• action, pump
• action, recoil operated
• action, retarded blowback
• action, rolling block
• action, semiautomatic
• action, side lock
• action, simple blowback
• action, single shot
• action, slave
• action, slide
• action, sliding block
• action, straight pull bolt
• action, stripped
• action, tip-up
• action, top-break
• action, trap door
• action, trombone
• action, turn-bolt
• action, under-lever
• double-action
• single-action

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.