An air rifle is a pneumatic rifle which fires projectiles using compressed air or other high pressure gas as a propellant. There are different methods of powering an air rifle. These methods can be broadly divided into 3 groups - spring-piston, gas ram and pneumatic.
Spring-Piston Air Rifles:
Spring-piston air rifles are able to achieve muzzle velocities near the speed of sound from a single stroke of a cocking lever or the barrel itself. The difficulty of the cocking stroke is usually related to the power of the gun, with higher muzzle velocities requiring greater cocking effort.
Spring-piston air rifles operate by means of a coiled steel spring-loaded piston contained within a compression chamber, and separate from the barrel. Cocking the gun causes the piston to be compressed until it engages the sear; pulling the trigger releases the sear and piston allowing it to move, compressing the air in the chamber directly behind the pellet. Once the air pressure has risen enough to overcome any static friction and/or barrel restiction holding the pellet, the pellet moves forward, propelled by an expanding column of air. All this takes place in a fraction of a second, during which the air undergoes adiabatic heating to several hundred degrees during compression, and then cools as the air expands once more.
Modern air rifle lubricants are generally a compounded mix of ingredients, such as silicone paste and molybdenum disulfide. These compounds are designed to not burn at the temperatures reached in air rifle compression chambers.
Spring-piston air rifles seem to have a practical upper limit of 1200 ft/s (370 m/s) for .177 cal (4.5 mm) pellets. Higher velocities cause unstable pellet flight and loss of accuracy. Drag increases rapidly as pellets are pushed past the speed of sound, so it is generally better to increase pellet weight to keep velocities subsonic in high-powered rifles.
Most spring piston air rifles are single-shot breech-loaders by nature but multiple-shot rifles have been increasingly common in recent years. Spring guns are typically cocked by a mechanism requiring the gun to be hinged at the mid-point (called a break barrel), with the barrel serving as a cocking lever. Other systems that are used include side levers, under-barrel levers, and motorized cocking, powered by a rechargeable battery.
The better quality spring air rifles can have long service lives, often exceeding thirty years. Because they deliver the same energy on each shot, the trajectory is extremely consistent. This resulted in most Olympic air gun matches through the 1970s and into the 1980s being shot with spring-piston rifles.
Pneumatic Air Rifles:
Pneumatic-type air rifles require the pre-compression of air in a chamber prior to the gun being used. Similar to the spring type, they are cocked or charged using a lever. When fired, the compressed air in the chamber is used to force the projectile out. Due to this design, the amount of recoil is almost zero as there is no significant movement of mechanical parts during the firing cycle. These air guns may also be powered by CO2 stored in capsules or cylinders, and filled via a hand pump or dive tank.
Multi-Stroke pneumatic air rifles 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 1000 feet per second.
For beginners and intermediates, multi-stroke air rifles have been a cost-effective choice as they are generally the cheapest form of air rifle available. Several manufacturers make multi-stroke air rifles including, to name a few, Sheridan, Benjamin, Daisy, and Crosman. Modified multi-pump rifles, with stronger pump linkages and improved valves, can produce muzzle energies in excess of 30 foot-pounds from inexpensive rifles. Modification kits for Sheridan and Benjamin rifles are available from commercial suppliers.
Single-stroke air rifle is 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 air rifles 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 rifles have higher initial costs but very low operating costs compared to CO2 rifles.
PCP rifles 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. They are widely utilized in the sport of Field Target shooting, 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. One of the traditional rifles for hunting wolves in Russia was said to be a large-caliber reservoir air-rifle. It is said to have shot silently to avoid warning the pack, though such unsilenced air guns are typically loud. Modern reservoir guns in larger calibers (6mm to 9 mm) are often used for hunting small game in the U.S.
During the discharge cycle, the hammer of the rifle is released by the sear to strike the valve. This usually involves the hammer moving toward the rear of the rifle, unlike firearms where the hammer normally 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.
CO2 Air Rifles:
CO2 air rifles use a disposable cylinder, a powerlet, that is purchased pre-filled with 12 grams of liquefied carbon dioxide, although some, usually more expensive models, use larger refillable CO2 reservoirs like those typically used with paintball markers.
Carbon dioxide-powered rifles have two significant advantages over pre-charged pneumatic air rifles:
(1.) A simpler system for compact storage of energy - a small volume of liquid converts to a large volume of pressurized gas.
(2.) No pressure regulator. Within a temperature range tolerable to humans there is little need to regulate the inherently suitable pressure for low-to-moderate-power air rifles. The vapor pressure is dependent only on temperature, not tank size, as long as some liquid CO2 remains in the reservoir.
These two advantages allow CO2 rifles to be constructed more simply than rifles using a pressurized air reservoir. Some CO2-powered rifles have detachable or fixed reservoirs that are loaded with pressurized gas from a larger cylinder. Most CO2 powered rifles use the standard 12 gram Powerlet disposable cylinder invented by Crosman. Recently, the same company introduced a new 88 gram disposable air source cylinder that is used in some of their rifles.
Most CO2 powered rifles are relatively inexpensive, although there are still a few precision target rifles on the market that use CO2.
The CO2 system has been used in experimental non-lethal law enforcement weapons, where high power delivery systems launch rubber batons or bean bags out of a gas-powered launcher, much like a non-lethal shotgun system (but at lower velocities, thus being safer).