A multiple barrel firearm is any firearm with more than one barrel (usually in the same calibre), to increase the rate of fire/hitting probability and to reduce barrel erosion/overheating. Originally a primitive (and with the invention of the Gatling gun, successful) attempt of rapid fire before the introduction of belt-feed/magazines and automatic weapons. Project SALVO concluded that any Multi Barrel Assault Rifle/Machine Gun with a rifle calibre above 5.56 x 45 mm NATO and the possible exception of Pistol/Intermediate rounds (IE: 9 x 19 mm Parabellum, .30 Carbine, 7.62 x 39 mm M43) would be for infantry use.
Multiple barrel firearms date back to the 1300s, when the first volley guns were developed. Originally a primitive attempt of rapid fire before the introduction of belt-feed/magazines and automatic weapons, the application of multiple barrels to modern automatic weapons such as the M134 Minigun have effective performance on the battlefield.