A shotgun (also known as a scattergun, or historically as a fowling piece) is a long-barreled firearm designed to shoot a straight-walled cartridge known as a shotshell, which usually discharges numerous small pellet-like spherical sub-projectiles called shot, or sometimes a single solid projectile called a slug. Shotguns are most commonly smoothbore firearms, meaning that their gun barrels have no rifling on the inner wall, but rifled barrels for shooting slugs (slug barrels) are also available. Shotguns come in a wide variety of calibers and gauges ranging from 5.5 mm (.22 inch) to up to , though the 12-gauge (18.53 mm or 0.729 in) and 20-gauge (15.63 mm or 0.615 in) bores are by far the most common. Almost all are breechloading, and can be single-barreled, double-barreled, or in the form of a combination gun. Like rifles, shotguns also come in a range of different action types, both single-shot and repeating. For non-repeating designs, over-and-under and side-by-side break action shotguns are by far the most common variants. Although revolving shotguns do exist, most modern repeating shotguns are either pump-action or semi-automatic, and also fully automatic, lever-action or bolt-action to a lesser extent. Preceding smoothbore firearms (such as the musket) were widely used by armies in the 18th century. The muzzleloading blunderbuss, the direct ancestor of the shotgun, was also used in similar roles from self-defense to riot control. Shotguns were often favored by cavalry troops in the early to mid-19th century because of its ease of use and generally good effectiveness on the move, as well as by coachmen for its substantial power. But by the late 19th century, these weapons became largely replaced on the battlefield by breechloading rifled firearms shooting spin-stabilized cylindro-conoidal bullets, which were far more accurate with longer effective ranges. The military value of shotguns was rediscovered in the First World War, when American forces used the pump-action Winchester Model 1897s in trench fighting to great effect. Since then, shotguns have been used in a variety of close-quarter roles in civilian, law enforcement and military applications. The smoothbore shotgun barrel generates less resistance and thus allows greater propellant loads for heavier projectiles without as much risk of overpressure or a squib load, and are also easier to clean. The shot pellets from a shotshell are propelled indirectly through a wadding inside the shell and scatter upon leaving the barrel, which is usually choked at the muzzle end