Although the BB is a single type of round projectile made of steel, the number of types of pellets available is enormous. In general, pellets have a shape known as diabolo which means that the front and rear sections are of larger diameter than the middle. One reason for this is to reduce the friction that would result if more of the pellet made contact with the barrel. Even though some airguns are rather powerful, the amount of work necessary to push the pellet down the barrel would be large if the pellet had a shape like a bullet.
Another feature of pellets is that they have hollow bases (the skirt area of the pellet) that are generally larger in diameter than the front section (known as the head of the pellet). This enables the skirt to effectively seal the bore against the pressure pushing on the base of the pellet while the friction on the head area is not too high.
The pellet velocity from any air rifle depends on the weight of the pellet. Heavier pellets simply can not be driven as fast as lighter ones. However, heavier pellets normally retain their velocity better as a result of their having higher ballistic coefficients. Even though initial velocity is lower, a greater percentage of that velocity is retained down range. Although a great deal of emphasis is placed on high muzzle velocity, pests are normally shot at some distance from the muzzle so the ability of the pellet to retain velocity is also important.