A BB gun is a type of air gun designed to shoot metallic spherical projectiles called BBs (not to be confused with similar-looking bearing balls), which are approximately the same size as BB-size lead birdshot used on shotguns (0.180 in or 4.6 mm in diameter). Modern BB guns usually have a smoothbore barrel with a 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. Some manufacturers still make the slightly larger traditional lead balls that weigh around 0.48–0.50 g (7.4–7.7 gr), which are generally intended for use in rifled barrels (due to lead having better malleability and exerting less wear on riflings).

The term "BB gun" is frequently used incorrectly to describe airsoft guns, which shoot plastic pellets (also often referred to as "BBs") that are larger (usually 6 mm or 0.24 in in diameter) but much less dense than metal BBs, and have significantly lower ballistic performance. The term is also sometimes used to describe a pellet gun, which shoots diabolo-shaped (not spherical) lead projectiles at higher power and velocity. Although some BB guns can also shoot pellets, the reverse situation is not true: steel BB balls have greater stiffness and are not meant to be shot from pellet guns, whose barrels are typically rifled and thus can get stuck (similar to a squib load in firearms) and lead to a damage or mechanical failure within the pellet gun.