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A pellet is a non-spherical projectile designed to be fired from an air gun. Air gun pellets differ from bullets and shot used in firearms because of the pressures encountered: airguns operate at pressures as low as 50 atmospheres, while firearms operate at thousands of atmospheres. Airguns generally use a slightly undersized projectile that is designed to obturate upon shooting so as to seal the bore, and engage the rifling; firearms have sufficient pressure to force a slightly oversized bullet to fit the bore in order to form a tight seal. 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" (or "wasp waist pellet") is the most common design found today. It 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 when pressurized by the gun. 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 centred in the bore, while keeping the friction as low as possible. The effect of friction is used in order to keep the pellet stationary until the piston has reached the end of its travel, compressing as much air as is possible. The skirt of the pellet is thin, and made of a malleable material, usually lead or lead alloy, 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, whereby imparting 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, and is less accurate. In most cases of this type, the solid head in the front and hollow skirt in back will work to prevent the pellet from tumbling. However, this is not always the case, and some ammo types of this nature will tumble in flight, and hit the target sideways. When this happens the pellet will not leave a clean, round hole in the paper, but will instead, leave behind a keyhole-shaped silhouette. This phenomenon is known as keyholing.
Pellets are designed to travel at subsonic speeds. High velocities can cause light pellets to overly deform, or even break apart in flight. The transition from subsonic to supersonic velocities will cause almost all pellets to tumble. The closer a pellet gets to the speed of sound, the more unstable it becomes. This is a problem for high powered break-barrel and pre-charged pneumatic air rifles, which often can push a normal pellet to velocities exceeding the speed of sound. A few companies have addressed this issue by manufacturing heavier than normal pellets for use in these high powered air guns. The heavier weight of these pellets ensure that they will travel at speeds well below the sound barrier, resulting in less tumbling and more overall accuracy. Their weight also makes them less susceptible to air resistance, and thus imparts more kinetic energy downrange, increasing lethality.
Match shooting use
Match pellets are used for the 10 metre air rifle and 10 metre air pistol disciplines. These 4.5 mm (0.177 in) calibre pellets have wadcutter heads, meaning the front is (nearly) flat, that leave clean round holes in paper targets for easy scoring. Match pellets are offered in tins and more elaborate packagings that avoid deformation and other damage that could impair their uniformity.
Match pellets are made of soft lead (a lead alloy with low antimony content). The antimony content is used to control the hardness of the soft lead alloy. It is a very soft alloy, which makes it easy to process. Since the soft lead alloy is prone to strongly deform when striking a bullet catcher, it rapidly loses its kinetic energy and will not easily bounce off. Lead is toxic and hazardous to the environment, so that when shooting with lead pellets precautions should be taken.
Match air gun shooters are encouraged to perform shooting group tests with their gun clamped in a fixed rest in order to establish which particular pellet type performs best for their air gun. To facilitate maximum performance out of various air guns the leading match pellet manufacturers produce pellets in graduated weight variants (the light/high speed variants are often marketed for air pistol use) and with graduated "head sizes", which means the pellets are offered with front diameters from 4.48 mm (0.176 in) up to 4.52 mm (0.178 in).
However at higher and top competitive levels, even these variations are thought too coarse-grained and match pellets are batch tested; that is, the specific gun is mounted in a machine rest test rig and pellets from a specific production run on a specific machine with the same ingredients fed into the process (a batch) are test-fired through the gun. Many different batches will be tested in this manner, and the pellets which give the smallest consistent group size without fliers (shots which fall outside of the main group) will be selected (small but inconsistent group sizes are not useful to a top competitor); and the shooter will then purchase several tens of thousands of pellets from that batch. Group sizes of 4.5 mm (0.177 in) diameter are theoretically possible, but practically shot groups of 5.0 mm (0.197 in) are considered highly competitive. Unbatched ammunition, especially if the air gun is not regularly cleaned, is generally thought to be capable of only 8.0 mm (0.315 in) diameter group sizes. Batch testing match pellets for a particular gun is not generally thought to be worthwhile until the shooter reaches a high proficiency level (around the 95% level (570 for men, 380 for women).