A pellet is a non-spherical projectile designed to be fired from an air gun. Air gun pellets differ from bullets used in firearms because of the pressures encountered: airguns operate at pressures as low as 50 atmospheres, while firearms operate at thousands of atmospheres.
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.