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.