An air gun is a pneumatic gun which shoots projectiles by using compressed air or other high pressure gas as a propellant.
Modern air guns are usually low-powered because of safety reasons and legal limitations. They can drive a pellet beyond 330 m/s, approx. the speed of sound. Most low-powered air guns can be securely fired in a garden also. In some countries, it is illegal to use them in residential areas. Air guns can be highly accurate and are used in the Olympic Games.
There are different ways of supplying power to an air gun. They can be classified into 3 groups: spring-piston, pneumatic, and CO2.
Spring-piston: Spring-piston air guns can attain muzzle velocities close to the speed of sound from a single stroke of a cocking lever. The cocking stroke is related to the power of the gun with higher velocities require larger cocking effort.
Spring-piston guns are operated by a spring loaded piston within a compression chamber. Cocking the gun compresses the spring until a small hook engages the sear. When the trigger is pulled, the sear is released which decompresses the spring and the piston is pushed forward, compressing the air. When the air pressure increases enough to overcome friction, the pellet moves forward.
Spring guns have disadvantage of large recoil and spring vibrations. The vibrations can be controlled by adding spring guides or by tuning by air gun smiths.
Pneumatic: Pneumatic air guns make a use of pre-compressed air as the energy source. Single-stroke and multi-stroke guns use a pump for pressurizing air. These reservoirs are filled by using a high pressure hand pump (reaching pressures of 300 bar) or by transferring air from a diving cylinder. Hence, there is no major movement of heavy parts and recoil is only 'true recoil'.
Multi-Stroke pneumatic air guns require 2 to 10 pumps to store compressed air. Through this, variable power is possible and the user can change the power for different shooting ranges. The higher quality rifles can shoot to speeds above 1000 fps.
In single stroke guns, one motion of the cocking lever is sufficient to compress the air. The single pump system is usually used in target rifles and pistols, where the higher energy of a multi-stroke is not needed.
Pre-charged Pneumatic: Pre-charged pneumatic (PCP) air guns can be used for competition and hunting. Because of the cylinders or charging systems, PCP guns have larger initial cost but very low operating costs when compared to CO2 guns.
CO2: Most CO2 guns use a disposable cylinder, pre-filled with 12 grams liquefied CO2 and some expensive models use larger refillable CO2 reservoirs.
These guns have 2 key advantages over pre-charged guns: (1.) a small volume of liquid converts to a large volume of pressurized gas. (2.) No pressure regulator.