A pellet is the term for a non-spherical projectile designed to be fired from an air gun. Pellets differ from bullets used in firearms because of the difference in pressures encountered; firearms operate at pressures of thousands of atmospheres, while airguns operate at pressures as low as 50 atmospheres. Firearms have sufficient pressure to swage a slightly oversized bullet to fit the bore to form a tight seal, while airguns generally use a slightly undersized projectile that is designed to obturate upon firing to seal the bore and engage the rifling. Since pellets may be fired through the smoothbore BB gun, they are often designed to be inherently stable, much like the Foster slugs used in smoothbore shotguns.
The design called the "diablo" pellet is the most common design found today. The diablo pellet can have a flat, round, hallow point or pointed tip, followed by a taper to a thin waist. From the waist back, the pellet is hollow, and flares out to full diameter. The head, or solid part in front of the waist, is usually sized to fit the bore just touching the rifling. This keeps the pellet centered in the bore, while keeping the friction low. The skirt is thin and made of a soft, deformable material, usually lead, although some pellets use tin or even plastic. When fired, the skirt will obturate to fit the bore and provide a good seal, and engaging the rifling to provide spin. In a smoothbore barrel, the skirt will still flare to provide a tight seal, but since there is no rifling the pellet will not spin. In this case, the solid head in the front and hollow skirt in back will prevent the pellet from tumbling and provide some accuracy. Pellets are designed to travel at subsonic speeds. High velocities can cause light pelets to deform and the transition from supersonic to subsonic will cause most pellets to tumble.