Pneumatic

Pneumatic airguns utilize pre-compressed air as the source of energy to propel the projectile. Single-stroke and multi-stroke guns utilize an on board pump to pressurize the air in their reservoir, Pre-charged Pneumatic guns' reservoirs are filled using either a high-pressure hand pump (often capable of attaining pressures of 30 MPa) or by decanting the necessary volume/pressure of air from a diving cylinder. Because of this design, having no significant movement of heavy mechanical parts during the firing cycle, the recoil produced is only the "true" recoil, equivalent to the equal and opposite reaction to the pellet and air volume's acceleration up the bore.

Multi-stroke

Multi-Stroke pneumatic air guns require 2-10 pumps of an on-board lever to store compressed air within the air gun. Variable power can be achieved through this process, as the user can adapt the power level for long, or short-range shooting. The design of higher quality and match-grade multi-stroke air rifles can propel a pellet to speeds in excess of 1,000 feet per second (300 m/s).
For beginners and intermediates, multi-stroke air rifles have been a cost-effective choice as they are generally the cheapest form of air gun available. Several manufacturers make multi-stroke air guns including, to name a few, Sheridan, Benjamin, Daisy, and Crosman. Modified multi-pump guns, with stronger pump linkages and improved valves, can produce muzzle energies in excess of 30 foot-pounds force (41 J) (from inexpensive guns. Modification kits for Sheridan and Benjamin rifles are available from commercial suppliers.

Single-stroke

As the name implies, one motion of the cocking lever is all that is needed to compress the air for propulsion. The single-pump system is usually found in target rifles and pistols, where the higher muzzle energy of a multi-stroke pumping system is not required.

Pre-charged Pneumatic

Pre-charged pneumatic (PCP) airguns can be used for hunting and competition. These are usually filled by decanting from an air reservoir, such as a diving cylinder or by charging directly with a hand pump. Because of the need for cylinders or charging systems, PCP guns have higher initial costs but very low operating costs compared to CO2 guns. These guns are often used for hunting purposes in countries other than the U.S. because of restrictive firearm laws.
PCP guns were the very first air guns; an experimental gun was made for King Henry IV of France in 1600. The Austrian military issued air rifles designed by Girandoni to special troops in the late 18th and early 19th century.
PCP guns have very low recoil and can fire from fewer than 30, to as many as 500 shots per charge. The ready supply of gas, has allowed the development of semi-automatic PCP air guns. Though technology allows this design, these types of PCP airgun are not permitted in certain countries, e.g. the United Kingdom. PCP guns are very popular in the UK and Europe because of their accuracy and ease of shooting. They are widely utilized in the sport of Field Target shooting, [14] and fitted with telescopic sights.
PCP guns are frequently used for hunting. In some countries, the use of a sound moderator or silencer makes these rifles particularly quiet, an advantage for hunters. Modern reservoir guns in larger calibers (6 mm to 9 mm) are often used for hunting small game in the U.S.
Earlier hand pumps for charging carried with them problems of fatigue (both human and mechanical), temperature warping, and condensation. None of those is beneficial to good shooting or the longevity of the rifle. More modern design hand pumps with built-in air filtration systems overcome many of these problems. Using scuba-quality air decanted from a scuba cylinder provides a clean, dry, high-pressure air supply that is consistent and available at low cost.
During the discharge cycle, the hammer of the rifle is released by the sear to strike the valve. The hammer may move rearwards or forwards, unlike firearms where the hammer almost always moves forward. Prior to being struck by the hammer, the valve is held closed by a spring and the pressure of the air in the air gun's tank. The pressure of the spring is constant, and the pressure of the air changes with each successive shot. As a result, when the tank pressure is at its peak, the valve permits passage of less total volume of air than when the tank pressure has been reduced by a series of shots. This results in a somewhat greater consistency of velocity from shot to shot than would otherwise be expected, and accuracy with a rifle is mainly dependent on consistency.
The better PCP rifles and pistols are often regulated, i.e. the valve operates within a secondary chamber within which the air pressure is kept constant for a set number of shots, rather than directly within the main reservoir. The pressure within this secondary chamber is maintained at a lower pressure than the pressure in the main reservoir by means of a regulator. Thus shot to shot consistency is far greater than in an unregulated rifle, at least as long as the pressure in the main reservoir is higher or equal to the regulated pressure in the secondary chamber. Beyond this point, the rifle or pistol will operate as any unregulated gun, and velocities drop rapidly.
The PCP is valuable to the small game hunter, pest controller, dedicated target shooter, marksmanship instructor and any other who requires precision, rather than the firepower of a firearm.

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