BB gun
A BB gun is a type of air gun that fires metallic spherical projectiles called BBs (not to be confused with similar-looking ball bearings), which are approximately the same size as BB-size lead birdshot used in shotguns (0.180 in or 4.6 mm in diameter). Modern BB guns usually have a smoothbore barrel of 4.5 mm (0.177 in) caliber, and use steel balls that measure 4.3–4.4 mm (0.171–0.173 in) in diameter and 0.33–0.35 g (5.1–5.4 gr) in weight, usually zinc- or copper-plated for corrosion resistance.