BB guns are a type of air gun designed to shoot spherical projectiles called BBs after the shot pellet of approximately the same size. Modern BB guns usually have a barrel with a bore and caliber of 0.177 inches (4.5 mm) and are available in many varieties. BB shot for modern BB guns is usually steel, plated either with zinc or copper to resist corrosion, and measure 0.171 to 0.173 inches (4.3 to 4.4 mm) in diameter. Some manufacturers also still make lead balls of slightly larger diameter and which are generally intended for use in rifled BB gun barrels.

The term is often incorrectly used to describe a pellet gun. Although in many cases, a BB can be fired in a pellet gun,pellets usually cannot be fired in a gun specifically designed for BBs. But many air gun models allow for both types of ammunition.