BB guns are a type of air gun designed to fire usually spherical projectiles, called BBs after the Birdshot pellet of approximately the same size. They can also be pointed, but these are usually used for bird hunting. "BB" in this case stands for "Ballistic Ball". Modern day BB guns usually have a smoothbore barrel, with a bore diameter and caliber of 0.177 inches (4.5 mm). BBs for modern day bb guns are usually steel, plated either with zinc or copper to resist corrosion, and measure 0.171 to 0.173 inches (4.34 mm to 4.39 mm) in diameter. Some manufacturers also still make lead BBs of slightly larger diameter and which are generally intended for use in rifled bb gun barrels, as were formerly used in bb guns. Some Asian companies make plastic BBs for recreation.
Airsoft guns are also commonly referred to as BB guns or pellet guns, as they also shoot spherical projectiles through a smoothbore barrel. However, airsoft "BBs" are 6 mm in diameter (0.24 inches), made of plastic or other non-metallic materials, and are designed specifically to be non-lethal.