There are two main types of airgun ammunition: pellets and BBs.
Pellets are generally wasp-waisted cylinders made of lead or non-lead alloys and provide the highest accuracy in the rifled barrels of adult precision air rifles and air pistols. Pellets of various calibers are available in flat-nose, round-nose, pointed, and hollow-point configurations.
There are several factors to keep in mind when selecting an airgun caliber:
Accuracy — Every airgun will have a particular pellet that it likes and will produce the best accuracy. Having a wide spectrum of pellets increases the odds of finding a pellet that will work well in a particular airgun. If accuracy is the sole concern, .177 offers the greatest variety of pellets, with .22 would be close behind.
Weight – The lightest pellets (between 4 and 5 grains) available are .177, but it is rare to find a .177 pellet heavier than about 16 grains. By contrast, .25 caliber pellets are available as heavy as 34.9 grains and usually not lighter than 17.7 grains.
Speed and trajectory – Shot from the same airgun powerplant, a light pellet will generally fly faster than a heavy pellet. At any given velocity, however, a heavier pellet will carry more energy down range and will usually retain it longer than a light pellet that was launched at the same speed. Because of these considerations, for a really fast, flat trajectory out to, say, 50 yards or so, .177 would be a good choice. But for greater ranges, a bigger caliber with heavier pellets would be more appropriate.
Power and impact – Launched at equal velocities, a heavy pellet will typically deliver more foot-pounds of energy to the target than a light pellet. If velocity and accuracy are equal, for greater hitting power, chose the heaviest pellet and largest caliber.
Wound ballistics – Bigger pellets produce bigger holes, but smaller diameter pellets often penetrate deeper.
BBs are round balls generally made of steel (although lead BBs are available in some calibers).