The component of an airgun that provides the gas pressure needed to propel projectiles is called its
powerplant, which uses internally stored pressurized gas; and
compressed gas (most commonly
CO2), which uses external sources of pressurized gas.
Spring-piston A
spring-piston air gun (also known as a
spring gun or simply a "
springer") operates by means of a
spring-loaded
piston pump assembly contained within a compression chamber separate from the
gun barrel. Traditionally, a
grease-lubricated steel
coil spring is used as the powerplant main spring. Before shooting, the user needs to manually cock the gun by flexing a lever connected to the pump assembly, which pulls the pump piston rearwards and compresses the main spring until the rear of the
piston engages the
sear. When shooting, the
trigger is pulled and disengages the sear, allowing the main spring to decompress and release its stored
elastic potential energy, pushing the piston forward, thereby compressing the air in the pump cylinder. Because the pump outlet (located to the front of the pump) is directly behind the pellet seated in the barrel
chamber, once the air pressure has risen enough to overcome the
static friction and/or barrel restriction holding back the pellet, the pellet is propelled forward by an expanding column of pressurized air. All this takes place in a fraction of a second, during which the air undergoes
adiabatic heating to several hundred degrees and then cools as the air expands. This can also cause a phenomenon referred as "dieseling", where flammable substances in the compression chamber (e.g., petroleum-based lubricant) can be
ignited by the compression heat like in a
diesel engine, and lead to an
afterburner effect with (often unpredictable) additional thrusts. This often results in combustion smoke coming out of the
muzzle and potential pressure damage to the rubber
O-ring seals inside the gun. Dieseling can be made to occur
intentionally to increase power, by coating the pellet with lubricant or
petroleum jelly, although this may result in damage to the breech seal. Most spring-piston guns are
single-shot breechloaders by nature, but multiple-shot
repeaters with
magazine feeders have been introduced in recent years by manufacturers such as
Gamo,
Umarex and Hatsan. Spring-piston guns, especially the high-powered "magnum" guns, are able to achieve muzzle velocities exceeding the
speed of sound. The effort required for the cocking stroke is proportional to the designed power of the gun, with higher muzzle velocities requiring a stiffer spring and hence a greater cocking effort. Spring-piston guns have a practical upper limit of for
.177 cal (4.5 mm) pellets, as higher velocities cause unstable pellet flight and loss of accuracy. This is due to the extreme
buffeting caused when the pellet reaches and surpasses
transonic speed, then slows back down and goes through
sound barrier again, which is more than enough to destabilize the pellet's flight. Shortly after leaving the barrel, the supersonic pellet falls back below the speed of sound and the shock wave overtakes the pellet, causing its flight stability to be disrupted. Drag increases disproportionately as pellet velocity increases, so it is generally better to increase pellet weight to keep velocities subsonic in high-powered guns. Many shooters find that velocities in the range offer an ideal balance between power and accuracy. Spring guns are typically cocked by one of the following mechanisms: • Break-barrel — like a
break action firearm, the barrel is hinged at the junction with the
receiver and is flexed downwards to expose the
breech and serves as the cocking lever • Fixed-barrel – the barrel is fixed to the receiver, and the cocking is done with a separate cocking lever • Underlever – the cocking lever is located underneath the barrel or the receiver and is flexed downwards during cocking • Sidelever – the cocking lever is located to the side (usually the right side) of the receiver and is flexed sideways during cocking • Overlever – the cocking lever is located above the barrel and is flexed upwards during cocking, seen in some air pistols • Motorized cocking powered by a rechargeable battery (rare) Spring-piston guns, especially high-powered ("magnum") models, do still
recoil as a result of the mainspring pushing the piston forward. Although the recoil is less than that of some
cartridge firearms, it can make the gun difficult to shoot accurately as the spring recoil is in effect while the pellet is still within the barrel. Spring gun recoil also has a sharp forward component, caused by the piston hitting the front end of the pump chamber when the spring has fully decompressed. These rapid double-jerking movements are known to damage
scopes not rated for spring gun usage. In addition, the spring often has unpredictable collateral transverse vibrations as well as
torquing, both of which can cause accuracy to suffer. These vibrations can be controlled by adding features like close-fitting spring guides or by aftermarket tuning done by gunsmiths who specialize in air gun modifications, a common one being the addition of high viscosity tenacious grease to the spring, which lubricates and serves to dampen vibration. The better quality spring guns can have very long service lives, being simple to maintain and repair. Because they deliver the same mechanical energy output on each shot, external ballistics are quite consistent. Most
Olympic air gun competitions through the 1970s and into the 1980s were shot with spring-piston guns, often of the opposing-piston recoil-eliminating type. Beginning in the 1980s, guns powered by compressed/liquefied
carbon dioxide began to dominate the competition. Today, the guns used at the highest levels of competition are powered by
compressed air.
Gas spring Some newer generation air guns incorporate a
gas spring (commonly referred to as a
gas piston,
gas ram,
gas strut or
nitro piston) instead of a mechanical spring. The spring itself is essentially a stand-alone enclosed
piston pump without outlets and with pressurized air or
inert gas (such as
nitrogen) held tightly sealed within the
cylinder. When the gun is cocked, the gas inside the cylinder gets further compressed by the piston, stores
potential energy and acts as a
pneumatic accumulator. Gas spring units require higher precision to manufacture since they require a low-friction sliding seal that can withstand the high pressures within when cocked. The advantages of the gas spring include the ability to keep the gun cocked and ready to fire for extended periods of time without long-term spring
fatigue, no twisting
torque (caused by coil spring expanding) exerted onto the gun, smoother recoil pattern and faster "lock time" (the time between pulling the trigger and the pellet being discharged), which all result in more consistent accuracy. Gas springs perform more reliably in cold climates than coil springs, because metallic coil springs need to be coated with
lubricating grease which often thickens in low temperatures causing the gun to "freeze up". Gas springs also have less lateral and longitudinal vibrations than coil springs, hence are usually less "hold-sensitive", making it easier to achieve consistent
shot groupings.
Pneumatic pre-charged pneumatic air pistol, as used in
10 metre air pistol ISSF shooting events Pneumatic airguns propel the projectiles by utilizing the
pneumatic potential energy within
compressed air, which is pressurized beforehand and stored inside the gun, and then released through valves during shooting. Single-stroke and multi-stroke pump guns utilize an on-board
hand pump to pressurize air in an internal
reservoir tank, while pre-charged pneumatic guns' reservoirs are filled from an external source using either a high-pressure
stand pump or by decanting air from a larger
diving cylinder.
Pump pneumatic Pump pneumatic air guns, or
pump guns, use a lever-operated onboard air pump to pressurize an internal reservoir, and then discharge the stored compressed air during shooting. Depending on the design, pump guns can be either single-stroke or multi-stroke. • In
single-stroke pneumatic air guns (also known as "single pump") a single motion of the cocking lever is all that is required to mechanically compress the air. The single-pump system has always dominated the casual plinking market, and is usually found in target rifles and pistols, where the higher muzzle energy of a multi-stroke pumping system is not required. Single-stroke pneumatic rifles dominated the national and international
ISSF 10 metre air rifle shooting events from the 1970s to the 1990s, until being outperformed by the introduction of pre-charged pneumatic (PCP) guns (discussed
below). •
Multi-stroke pneumatic air guns (also known as "multi-pump") use multiple pumpings to achieve variable power levels in order to adapt for both long or short-range shooting. These air guns are usually single-shot, where each shot requires approximately 5 strokes. However, up to five shots are possible, usually requiring approximately 10 to 20 strokes, as long as the air reservoir is capable of storing higher pressure. For safety reasons, most multi-stroke guns are usually designed to have their pump lever jam when the reservoir has reached its maximum pressure limit, so the user can no longer pump the gun until it's discharged. The maximum pressure limit for the reservoir is approximately 20 to 30 strokes.
Pre-charged pneumatic Pre-charged pneumatic air guns or
PCP have their internal reservoir pre-filled from an external air source (such as a
diving cylinder or
air compressor, or by manual charging with a high-pressure
stand pump), and remain pressurized until depleted after repeated shooting. During shooting, the hammer strikes the reservoir's release valve, allowing a set volume of the pressurized air to be discharged into the chamber and propel the projectile. Depending on the release valve design, PCP air guns can be categorized into two types –
unregulated and
regulated (which has either a mechanical or electronic
regulator valve). In addition to
compressed air, custom PCP models using compressed pure
nitrogen and even compressed
helium have also been recently developed. variant of a
Walther LG400 PCP Air Rifle. Because of the need for cylinders or charging systems, PCP guns have higher initial costs but much lower operating costs when compared to CO2 rifles, and have superior performance over ordinary pump guns. Having no significant movement of heavy mechanical parts during the discharge cycle, PCP airgun designs produce lower
recoil, and can shoot as many as 100 shots per charge depending on the tank/reservoir size. The ready supply of air has allowed the development of semi- and fully automatic air guns. PCP guns are very popular in
Europe because of their accuracy and ease of use. They are widely utilized in
ISSF 10 meter air pistol and rifle shooting events and the sport of
Field Target shooting, and are usually fitted with
telescopic sights. Early stand pump designs encountered problems of fatigue (both human and mechanical), temperature warping, and condensation – none of which are beneficial to accurate shooting or the airguns' longevity. Modern stand pumps have multi-stage chambers and built-in air filtration systems and have overcome many of these problems. Using
scuba-quality air decanted from a diving cylinder provides consistently clean, dry, high-pressure air. During the typical PCP's discharge cycle, the hammer of the rifle is released by the sear to strike the
bash valve. The hammer may move rearwards or forwards, unlike firearms where the hammer almost always moves forward. The valve is held closed by a spring and the pressure of the air in the reservoir. The pressure of the spring is constant, and the pressure of the air released (which is also known as the
working pressure) decreases with each successive shot. As a result, when the reservoir pressure is high, the valve opens less fully and closes faster than when the reservoir pressure is lower, resulting in a similar total volume of air flowing past the valve with each shot. This results in a degree of partial self-regulation that gives a greater consistency of velocity from shot to shot, which corresponds to the middle "plateau" phase of the gun's shot-to-shot
muzzle velocity profile (also known as the
power curve ). Well-designed PCP will display good shot-to-shot consistency over a long period, as the air reservoir is being depleted. Other PCP rifles and pistols are regulated, i.e. the firing valve operates within a secondary chamber separated from the main air reservoir by the regulator body. The regulator maintains the pressure within this secondary chamber at a set pressure (lower than the main reservoir's) until the main reservoir's pressure drops to the point where it can no longer do so. As a result, shot-to-shot consistency is maintained for longer than in an unregulated rifle, and the gun can also output more shots due to reduced waste of reservoir pressure.
Compressed gas cylinders
Compressed gas guns, also known as
CG guns, are essentially pneumatic airguns utilizing detachable
pressure reservoirs in the form of prefilled external
gas cylinders (often with built-in
regulator valves), and are commonly referred to as
CO2 guns due to the ubiquitous commercial use of
bottled liquid carbon dioxide. However, more recent high-end models sometimes use larger
compressed air/
nitrogen (known as HPA or "N2") cylinders, which have higher operating pressures and better thermal stability.
CO2 CO2 pistol with adapted silencer
CO2 guns, like other pneumatic guns using compressed air, offer power for repeated shots in a compact package without the need for complex pumping or filling mechanisms. The ability to store power for repeated shots also means that repeating arms are possible. There are many replica revolvers and semi-automatic pistols on the market that use CO2 power. Most CO2 guns use a disposable cylinder called a "
Powerlet" cartridge, that is often purchased with of pressurized CO2 gas, although some, usually more expensive models, use larger refillable CO2 reservoirs like those typically used with
paintball markers. CO2 guns are popular for training, as the guns and ammunition are inexpensive, relatively safe to use, and no specialized facilities are needed for safety. In addition, they can be purchased and owned in areas where firearms possession is either strictly controlled or banned outright. Most CO2 powered guns are relatively inexpensive, and there are a few precision target guns available that use CO2.
High-pressure air The
high-pressure air (HPA) system, or
N2, was originally developed for
paintball markers as a replacement for CO2 cartridges, and uses powerlet interface-compatible
diving cylinders filled with either pure
nitrogen or
compressed air (which is 78% nitrogen). Because nitrogen is more
inert than CO2, it remains in the
gaseous state longer when compressed; when it expands, it cools due to the
Joule-Thomson effect, but at a far lower rate than
liquid CO2 because there is no change in
state from liquid to gas. This
thermostability reduces the variation in output pressure associated with rapid successions of firing cycles, improving accuracy and reliability in extreme climates. However, because compressed air is stored at higher pressures (up to ) than liquid CO2 (stored at around ), HPA cylinders are more expensive. Cylinders smaller than may not even last as long as a standard CO2 cartridge when subjected to frequent use. It is also possible to power an HPA-compatible gun directly from a plug-in
air compressor without using a gas cylinder, though this type of setup is rare.
Air cartridge system First developed in the
United Kingdom during the 1980s as the Saxby-Palmer Ensign, and then by Brocock as the "Brocock Air Cartridge System" (BACS) and later into the "Tandem Air Cartridge" (TAC), this variation of the pre-charged pneumatic design have no pressure reservoir built into the gun, but instead use removable and reusable metallic
gas cylinders often known as "Brocock cartridges" after its British manufacturer. Each air cartridge is essentially a self-contained gas reservoir housed inside a
cartridge case (usually in the size of a
.38 Special), with an internal
bash valve designed to release the content when the base of the cartridge is struck. Prior to shooting, each cartridge is pre-filled with sufficient
compressed air for one shot (typically via a multi-stroke
hand pump) and has a
.177/
.22 caliber pellet pre-seated to its front slot, and the entire cartridge is then inserted into a
chamber in the
gun barrel. When shooting, the
hammer hits the rear of the air cartridge, mimicking the function of a
centerfire ammunition. This allows the airgun to be constructed and operated in similar fashions to genuine firearms. It also allows for higher shot consistency because each cartridge can be easily filled to an identical air charge, essentially removing the "power curve" of conventional PCP guns and bypassing any need for
regulators. It also simplifies the magazine feed design and eliminates the risk of deforming the projectiles when the
action seats each pellet, as the soft lead pellet is protected from contact with the action probe by the harder cartridge casing. The air cartridge system, both in the
rifle and
revolver forms, was at the peak of its popularity throughout the 1990s. The popularity of the Brocock range was such that, by 2002, estimates put the number of air cartridge guns in circulation around 70,000-80,000. and David McCrone, firearms adviser to the Association of Chief Police Officers and Deputy Chief Constable of
Greater Manchester Police, told the BBC's
Newsnight "there is evidence which would justify banning them". After the
UK government implemented the
Anti-social Behaviour Act 2003 in January 2004, it became illegal to possess such airguns without a
firearm certificate in the United Kingdom. Brocock subsequently ceased production of all air cartridge systems and turned to focus primarily on PCP airguns. ==Ammunition==