semi-auto shotgun

A semi-automatic firearm, also called self-loading firearm or autoloading firearm (though fully automatic and selective fire firearms technically are also self-loading), is one whose action mechanism automatically loads a following round of cartridge into the chamber (self-loading) and prepares it for subsequent firing, but requires the shooter to manually actuate the trigger in order to discharge each shot. Typically, this involves the weapon's action utilizing the excess energy released during the preceding shot (in the form of recoil or high-pressure gas expanding within the bore) to unlock and move the bolt, extracting and ejecting the spent cartridge case from the chamber, re-cocking the firing mechanism, and loading a new cartridge into the firing chamber, all without input from the user. To fire again, however, the user must exert effort to actively release the trigger, allow it to "reset", before pulling the trigger again to fire off the next round. As a result, each trigger-pull only discharges a single round from a semi-automatic weapon, as opposed to a fully automatic weapon, which will shoot continuously as long as the ammunitions are replete and the trigger is kept depressed.
Ferdinand Ritter von Mannlicher produced the first successful design for a semi-automatic rifle in 1885, and by the early 20th century, many manufacturers had introduced semi-automatic shotguns, rifles and pistols.
In military use, self-loading rifles were barely used in World War I, and most armies in World War II also still relied upon the manually loaded bolt-action rifles, with the exception of the United States, who in 1937 had adopted the M1 Garand as the standard-issue infantry weapon.

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