A BB gun is a pneumatic gun usually powered by compressed gas, electric revolution, or a spring. Most BB guns are shaped like normal guns but can look like toys. Some have reservoirs to store ammunition, some are breechloaders, and others are muzzleloaders. They use small round balls called BB's made of plastic or copper, usually .177 caliber (4.5 mm). Some also use shaped pieces of lead called pellets. These vary widely in shape, but generally have a flared back end for stability in flight and either a sharp point or blunt tip.

Copperhead is a common brand of BB gun ammunition. Two well-known American manufacturers of BB guns are Crosman and Daisy.