The Ballistic Steel BB’s (1500) are compatible with all 4.5mm Pistols BB guns are a type of air gun designed to fire spherical projectiles similar to shot pellets of approximately the same size. BB shot for modern BB guns is usually steel, plated either with zinc or copper to resist corrosion, and measure 4.3 to 4.4 mm (0.171 to 0.173 in) in diameter. Some manufacturers still make lead balls of slightly larger diameter which are generally intended for use in rifled BB gun barrels.