Airsoft ammo is called BB. The initials "BB" is believed to have derived from the words "ball bearing", which refers to the .17–.18 metal balls first used in BB-guns. Airsoft ammo are 6 mm and 8 mm plastic BBs. Ammo is categorized by weight and other factors that we'll briefly touch on. Airsoft BBs are weighed in grams, which is a metric unit of measurement. The range of BB weight includes, but is not limited to .12 g (lightest, primarily a pistol round), .15 g, .20 g (most commonly used), .25 g, .30 g (popular sniper round), and .43 g.
Other factors to consider when choosing your Airsoft BBs is quality. Cheaply made BBs have burrs, inconsistent shape and are poorly balanced, some even break inside the Airsoft gun. If you're at all concerned about accuracy, a predictable trajectory and consistent performance stay away from cheap ammo. Some signs of quality ammo include highly consistent spherical standards, no burrs and a polished finish. Crosman BBs meet the demands of high-end Airsoft gamers.
Other options on the market include biodegradable BBs. Also available are paint filled marker BBs (usually used in pistols or low powered Airsoft rifles) and marker BBs that have an exterior powder coating.