Pellet (air gun)
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An air gun pellet is a non-spherical projectile designed to be fired from an air gun.
They differ from bullets used in firearms because of the 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 shooting to seal the bore and engage the rifling. Since pellets may be shot through a smoothbore barrel, they are often designed to be inherently stable, much like the Foster slugs used in smoothbore shotguns.
The "diabolo", sometimes called wasp waist pellet is the most common design found today. The diablo pellet can have a flat, round, hollow 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 non-toxic alternatives are available that use tin or even plastic. When shot, the skirt will obturate to fit the bore and provide a good seal, and engage the rifling, thereby providing a 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 pellets to deform or even break apart and the transition from supersonic to subsonic will cause most pellets to tumble. The closer a pellet gets to the speed of sound the more unstable it becomes. A few companies have addressed this issue by manufacturing heavier than normal pellets for use in high powered air guns. These pellets travel at a reduced speed because of their weight and can be more accurate.
