Semi-automatic (military)

A semi-automatic firearm loads and fires one round with each pull of the trigger, in contrast to a full-automatic, which will continue to load and fire projectiles (i.e., bullets or shotgun loads) as long as the trigger is kept pulled and there is ammunition. In the context of individual weapons, while the term is sometimes used to suggest an especially powerful weapon, a semiautomatic pistol of a given caliber has a comparable rate of fire to a revolver of the same caliber. The only potential difference is that semiautomatics, which are fed from a magazine rather than a rotating cylinder, may have greater ammunition capacity: while a revolver rarely has more than six rounds, semiautomatics commonly have eight, and, with extended magazines, may have as many as 30.

There is significance, however, when speaking of warship guns and artillery that require loading of each round to be fired. More modern weapons of this type use fixed ammunition, which is similar to an enlarged version of a rifle or pistol cartridge, with projectile, propellant, and primer in one sealed package. Some large cannon, mostly naval in the twentieth century, require the projectile to be loaded separately from the propellant. This does allow precise control of the amount of propellant and thus the range of the trajectory, but results in a slower rate of fire.

The three cruisers of the Des Moines class, the last all-gun warships built by the United States, had nine 8-inch/203mm semi-automatic guns, mounted in triple turrets. For gunfire support of troops ashore, such a vessel had a far higher rate, and thus density, of fire than ships with manually loaded guns of the same size. During the Vietnam War, the USS Newport News, a cruiser of this class, is remembered for gun battles with North Vietamese shore batteries.