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Air gun

"Air rifle" and "Air pistol" redirect here. For other uses, see Air gun (disambiguation)

An air gun (air rifle or air pistol) is a rifle or pistol which fires projectiles by means of compressed air or other gas, (in contrast to a firearm which uses an explosion or combustion of a propellant). Air guns are used for hunting, leisurely recreational shooting (commonly known as plinking), and precision sports, such as the Olympic 10 m Air Rifle and 10 m Air Pistol events.

Most air guns use metallic projectiles as ammunition. Air guns that only use plastic projectiles are further classified as airsoft guns.

History

Air guns represent the oldest pneumatic technology, having existed since the 15th century. At that time, they had compelling advantages over the primitive firearms of the day. For example, air guns could be fired in wet weather and rain (unlike matchlock muskets) and with greater rapidity than muzzle-loading guns. Moreover, they were quieter than a firearm of similar caliber, had no muzzle flash, and were completely smokeless, thus not disclosing the shooter's position. Black powder muskets of the 18th and 19th century produced huge volumes of dense smoke when fired, a disadvantage compared to air rifles.

At the time, airguns posed a serious alternative to powder weapons. Although much more expensive, they were generally far superior. Robust air reservoirs had evolved with increasing technology, thereby improving the charge capacity while minimizing any possibility of bursting. Similarly, improvements in valve designs began to create well sealed chambers.

During this period, France, Austria and other nations had special sniper detachments using air rifles. The Austrian 1780 model was named Windbüchse (literally "wind rifle" in German). The gun was developed in 1778 or 1779 by the Tyrolese watchmaker, mechanic and gunsmith Bartholomäus Girandoni (1744-1799) and is sometimes referred to as the Girandoni Air Rifle or Girandoni air gun in literature (the name is also spelled "Girandony"or "Giradoni") or "Girardoni" . The Windbüchse was about 4 ft (1.2 m) long and weighed 10 pounds (4.5 kg), which was about the same size and mass as a conventional musket. The air reservoir was a removable, club-shaped butt. The Windbüchse carried twenty .51" (13 mm) lead balls in a tubular magazine. A skilled shooter could fire off one magazine in about thirty seconds, which was a fearsome rate of fire compared to a muzzle loader. A shot from this air gun could penetrate a one-inch wooden board at a hundred paces, an effect roughly equal to that of a modern 9 mm or .45" caliber pistol.

Around 1820, the Japanese inventor Kunitomo Ikkansai developed various manufacturing methods for guns, and also created an air gun based on the study of Western knowledge ("rangaku") acquired from the Dutch in Dejima.

Air guns appear throughout other periods of history. The celebrated expedition headed by Lewis and Clark (1804) reportedly carried a .42" (10 mm) reservoir air gun, believed to be produced by Girandoni. It held 22 round balls in a tubular magazine mounted on the side of the barrel. The butt stock served as the air reservoir and had a working pressure of 800 PSI. The rifle was said to be capable of 22 aimed shots in one minute.

During the 1890s, air rifles were used in Birmingham, England for competitive target shooting. Matches were held in public houses, which sponsored shooting teams. Prizes, such as a leg of mutton for the winning team, were paid for by the losing team. The sport became so popular that just after the turn of the 19th century, a National Air Rifle Association was created. During this time over 4000 air rifle clubs and associations existed across Britain, many of them in Birmingham. During this time, the air gun was associated with poaching because it could deliver a shot at a relatively quiet level.

Today's modern air guns are typically low-powered because of safety concerns and legal restrictions; however, high-powered designs are still used for hunting. These air rifles can propel a pellet beyond 1100 ft/s (330 m/s), approximately the speed of sound and produce a noise similar to a .22 caliber rimfire rifle. Using lead pellets, some current spring powered .177 pellet guns can break the sound barrier. Most low-powered airguns can be safely fired in a backyard or garden, and even indoors, with the proper backstop.

In some countries, air guns are still classified as firearms, and as such it may be illegal to discharge them in residential areas. Air guns can be highly accurate and are used in target shooting events at the Olympic Games, governed by the International Shooting Sport Federation (ISSF).

Legal issues

The legal definition of an air gun differs from country to country. In the United Kingdom, for example, air pistols generating more than 6 ft·lb_f (8.1 J) or air rifles generating more than 12 ft·lb_f (16.2 J) of energy are considered firearms.

Similarly, in Italy, any mechanism that produces a muzzle energy higher than 1 Joule and lower than 7,5 Joules is considered a "Low-power air gun"; the sale of such instruments is open to anyone over 18 years of age without license or registration required, but it can take place only in authorized gun shops whose owner must require the purchaser to display his ID Card as a proof of age and take note of its data for his registers; while any device developing a muzzle energy equal to, or higher than, 7,5 Joules is considered an "High-power air gun", requiring Police licensing and registration for purchase and detention just like any firearm; bows, crossbows and similar are exempt from this rule. The muzzle energy of such devices is certified by a governance office called "Banco di Prova". Air guns developing less than 1 Joule of muzzle energy are categorized as Airsoft, which are considered by law as toys, with no restriction whatsoever to their trade, except that them can never be modified to achieve an higher muzzle energy and must be only able to shoot 6mm BB plastic pellets.

In Canada, air guns with a muzzle velocity of over 500 ft/s (150 m/s) and a muzzle energy in excess of 4.2 ft·lb_f (5.7 J) are classified as firearms and must be registered, transported, stored, and used as such. Some air guns are prohibited in Canada, including replica air guns designed to resemble a real firearm and any air pistols with a barrel length of 4.13 inches (105 mm) or less. Prohibited replica air guns owned before December 1, 1998 are grandfathered. Persons committing a crime in Canada with an air gun face the same penalties as if they had committed the crime with a regular firearm.

In Japan, any air gun that fires a metallic projectile is restricted as a firearm, so only airsoft type guns are readily available.

Although it is illegal for U.S. cities to do so, some have attempted to restrict air gun sales and possession, usually regardless of the muzzle energy; these include: New York, New York; Camden and Newark, New Jersey; Michigan; Chicago and Morton Grove, Illinois; San Francisco, California; and Philadelphia, Pennsylvania.

Australian law classifies air guns and BB guns as "Category A" firearms, placing them in the same class as break-action shotguns and rimfire rifles, requiring a licence for ownership. Anyone found in Australia possessing an unlicensed airsoft pistol faces the same charge as a person who unlawfully possesses a firearm. In many other areas of the world, however, air guns are not considered firearms and are not subject to regulation.

There are minimum ages for possession, usually 18 and sales of both air guns and ammunition may be restricted. Some areas may require permits and background checks similar to those required for firearms possession. In the UK, Brocock Air Cartridge System air guns, which use a pre-charged, single shot air cartridge (similar in size to a .38 Special cartridge) were banned after some pistols recovered by the police, were found to have been converted by criminals making them capable of firing rimfire ammunition or even .38 Special ammunition.

Due to further legislative restrictions in the UK, the online or mail order sales of new air guns, must be finalised 'face-to-face'; either at the retail store of purchase, or with a Registered Firearms Dealer (where an item may be shipped and the transfer completed). The sale and transfer of second-hand airguns is not affected by these new restrictions.

Finland is contemplating mandatory licensing of high powered airguns.

Air gun power sources

There are different methods of powering an air gun. These methods can be broadly divided into 3 groups - spring-piston, pneumatic, and CO₂. These methods are used in air rifles and air pistols.

Spring-piston

Spring-piston air guns 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 guns 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 assembly to compress the spring until a small hook on the rear of the piston engages the sear; pulling the trigger releases the sear and allows the spring to decompress, pushing the piston forward, thereby 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 restriction 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.

Spring-piston guns 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 guns. Many shooters have found that velocities in the 800 - range offer an ideal balance between power and pellet stability.

Most spring piston guns are single-shot breech-loaders by nature but multiple-shot guns have become more 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.

Spring guns, especially high-powered ones, have significant recoil resulting from the forward motion of the piston. Although this recoil is less than that of a cartridge firearm, it can make the gun difficult to shoot accurately as the recoil forces are well under way while the pellet is still traveling down the barrel. Most guns seem to respond well to a light, repeatable grip that allows the gun to vibrate the same way from shot to shot. Spring gun recoil also has a sharp forward component, caused by the piston as it hits the forward end of the chamber when the spring behind it reaches full expansion. This sudden forward acceleration helps to counteract the recoil, since the recoil and 'forward recoil' forces happen within milliseconds of each other, but it is infamous for the loosening and / or breaking of lenses and reticles found in low and medium priced telescopic sights. On any, but low powered spring guns, mounted telescopic sights should be air gun rated.

Spring guns can also suffer from spring vibrations that reduce accuracy. These vibrations can be controlled by adding features designed into the gun, like close-fitting spring guides, or by aftermarket tuning done by 'airgunsmiths' who specialize in air gun modifications. A common modification is the addition of viscous silicone grease to the spring, which both lubricates it and damps out vibration.

The better quality spring air guns can have very long service lives, being simple to maintain and repair. 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 guns, albeit often of the opposing-piston recoil-eliminating type. Beginning in the 1980s, guns powered by compressed, liquefied carbon dioxide began to dominate competition. Today, the guns used at the highest levels of competition are powered by compressed air stored at very high pressures of 2000 to 3000 lb/in² (14 to 21 MPa).

Gas 'Spring'

Some makes of air rifle (e.g. Weihrauch, Theoben) incorporate a gas spring instead of a mechanical spring. Pressurized air or nitrogen is held in a special chamber built into the piston, and this air is further pressurized when the gun is cocked. It is, in effect, a gas spring commonly referred to as a "gas ram" or "gas strut". Gas spring rifles require higher precision to build, since they require a low friction sliding seal that can withstand the high pressures when cocked. The advantages of the gas spring include the facility to keep the rifle cocked and ready to fire for long periods of time without harming the mechanism. Also, since there is no spring (and therefore a reduction in moving mass during firing) there is less (although some say slightly sharper), recoil. There is also an elimination of the associated problems of long-term spring fatigue and a faster "lock time" (the time between pulling the trigger and the pellet being discharged). The improvement in lock time makes for better accuracy since there is less time for the gun to move off target.

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 .

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 (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, 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 (5 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.

CO₂

Most CO₂ guns 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 guns have two significant advantages over pre-charged pneumatic air guns: (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 guns. 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 guns to be constructed more simply than guns using a pressurized air reservoir. Some CO₂-powered guns have detachable or fixed reservoirs that are loaded with pressurized gas from a larger cylinder. Most CO₂ powered guns use the standard 12 gram Powerlet disposable cylinder invented by Crosman. Recently, the same company introduced a new 88 gram disposable AirSource cylinder that is used in some of their guns.

On the other hand, liquefied CO₂ must be purchased, which introduces an element of cost that does not factor with a PCP gun/hand pump combination using "free" air, or is at least considerably lower when refilling from a diver's tank.

Furthermore, the pressure of gaseous CO₂ at ordinary ambient temperatures is only around 850–1000 psi (6 to 7 MPa), which is only a third of the safe working pressure of a typical full PCP reservoir (20 MPa or 2900 psi or more). The effect of this is that generally speaking CO₂ guns are lower powered and less efficient than PCP guns, which is why CO₂ guns are usually pistols or semi-target type rifles, with few guns (none of commercial note) reaching even the ·lbf (16.2 joules) licence-free energy limit for air rifles imposed in the UK.

CO₂ guns, like compressed air guns, offer power for repeated shots in a compact package without the need for complex cocking 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. These guns are popular for training, as the guns and ammunition are inexpensive, 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, although there are still a few precision target guns 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).

Safety

For safety, CO₂ containers must be kept at temperatures below 120 °F (49 °C) ; at temperatures above this level, the pressure begins to increase very rapidly, and can cause the container to fail. CO₂ containers with diameters at or above two inches (50 mm) have a pressure release "rupture" mechanism to release the contents over a certain pressure level and avoid explosion because of high temperature. These disks are generally calibrated to a minimum pressure corresponding to the level at 100% of the rated CO₂ capacity. Elevated temperatures, even those below the critical temperature, can cause increased leaking through seals.

Operating considerations

Re-filling Forcing more carbon dioxide gas into a reservoir of liquid and gas CO₂ while maintaining a constant temperature would not raise the pressure but merely convert the additional gas into liquid. By chilling the vessel to be filled, the lower vapor pressure will pull CO₂ from the source container. While the pressure in the reservoir is generally dependent only on the temperature, if the bottle is too full, that changes. The expansion of the liquid CO₂ will take up all the space in the bottle, preventing evaporation. At this point, the pressure increase with temperature becomes dangerously high.
Cooling Each time the gun is fired there is some evaporation of liquid to gas which is an endothermic process in which the pressure drops until enough ambient heat is absorbed to restore the pressure. When shooting at a rate faster than the cylinder can absorb heat from the environment to counter the cooling of the evaporating liquid, the pressure will drop, and the velocity is likely to drop as well in a non-regulated gun.

Calibers

The most common Air gun calibers are:

.177" (4.5 mm) - the most common caliber, also used in ISSF shooting events at the Olympic Games, it has the flattest trajectory of all the calibers for a given energy level, making accuracy simpler.
.20" (5.0 mm) - found in some European air guns and those manufactured by US air gun manufacturer, Sheridan. This is generally considered to be a 'compromise calibre', having a flatter trajectory similar to the .177 but more energy retention. .20" pellets tend to be of a similar weight to .22" pellets, but with an elongated profile, resulting in a higher Ballistic Coefficient (BC) than either .177" or .22" pellets.
.22" (5.5 mm & 5.6 mm) - the most common caliber for hunting small game, as it delivers large amounts of energy on impact (more so than the smaller calibers).
.25" (6.35 mm) - the largest commonly available caliber. This caliber is renowned for its impact, having the most energy retention of all calibers. However, it does have a highly parabolic trajectory at low energy levels and is thus more suited for higher powered rifles.

Custom airgun manufacturers regularly produce air rifles in common muzzleloading rifle calibers too, such as .45" (11.25 mm), .50" (12.5 mm), .58" (14.5 mm) and larger.

Ammunition

Pellet

The most popular ammunition used in rifled air guns is the lead diabolo pellet. This waisted projectile is hollowed at the base and available in a variety of head styles. The diabolo pellet is designed to be drag stabilized, though is not as stable as some other shapes in the transonic region. Pellets are also manufactured from tin, or a combination of materials such as steel-tipped plastic.

While some high-power Spring-piston air rifles can propel lightweight pellets at, or beyond, the speed of sound, accuracy may suffer and there may be a decrease in the working-life of the rifle's spring and piston seals. This is due to the pellet exiting the barrel before maximum (barrel) pressure has been reached, resulting in a high speed collision between the piston and the end of the air chamber.

Most air guns are .177 (4.5 mm) or .22 (5.5 mm / 5.6 mm), and are designed for target practice, small game hunting and field target shooting. Cost per round is less than $0.02 (US) for Olympic-quality ammunition, and far less for cheaper grades. Though less common, .20 and .25 caliber (5.0 mm and 6.4 mm) guns also exist and are used predominantly for hunting.

BB

The BB was once the most common air gun ammunition in the USA. A BB is a small ball, typically made of steel with a copper or zinc plating, of 4.4 mm/.175" diameter. Lead "Round Balls" are manufactured in numerous calibers too, however these are often 4.5 mm/.177" diameter and designed for use in .177 caliber rifled guns normally used for shooting pellets. Steel BBs can be acceptably accurate at short distances when fired from properly designed BB guns with smoothbore barrels.

Due to the hardness of the steel, they can not "take" to rifled barrels, which is why they are undersized (4.4 against 4.5 mm) to allow them to be used in .177" rifled barrels which when used in this configuration can in effect be considered smoothbore, but with a poorer gas-seal. Were they 4.5 mm diameter, they would jam in the bore. Therefore BB's lack the spin stabilization required for long-range accuracy, and usage in any but the cheapest rifled guns discouraged not least because the steel-to-steel contact may cause accelerated wear to the rifling's lands.

Typically BBs are used for indoor practice, casual outdoor plinking, training children, or for air gun enthusiasts who like to practice, but cannot afford high-powered air gun systems that use pellets. Some shotgunners use sightless BB rifles to train in instinctive shooting. Similar guns were also used briefly by the United States Army in a Vietnam-era instinctive shooting program called "Quick Kill" (Time magazine, Friday, July 14, 1967).

Choosing an air gun

The following table provides general guidelines for choosing an air gun for a desired feature.

Feature	Airgun Types	Comments
Self-Containment	spring-piston, multi-stroke pneumatic, single-stroke pneumatic	These guns require no additional CO₂ cylinders or external pumps, and are thus cheaper to operate.
Low Noise	spring-piston	Absence of loud gas discharges makes these guns quieter to operate. Consider when practicing in cramped urban areas. Where legal, a good quality suppressor (commonly referred to as a silencer or moderator) can make other types as quiet as a piston rifle.
Accuracy	pre-charged, single-stroke pneumatic, recoilless piston	Without the variable factor introduced by CO₂ vapor pressure or the recoil introduced by the spring, the mechanisms in these guns have more repeatable shots.
Convenience	pre-charged or CO₂ powered	These guns don't require constant cocking, and are hence more popular with recreational shooters. They are generally more expensive to operate.

While the above generalizations are helpful, the performance of an air gun will also depend on its quality. For example, a match-grade CO₂ rifle will have better accuracy than a cheaper spring-piston gun. The extra costs of a more expensive gun may translate into higher quality, tighter tolerances, and better accuracy.

It is also important to consider where (e.g. club range, backyard, farm) and how (e.g. competition, target practice, plinking, pest-control) an air gun will be used.