Air gun

An air gun ( airgun / pellet gun) is a gun that propels projectiles by means of compressed air or other gas, in contrast to firearms which use a propellant charge. Both air rifles and air pistols typically fire metallic projectiles, either pellets or BBs.

History

Air guns represent the oldest pneumatic technology. The oldest existing mechanical air gun, a bellows air gun dating back to about 1580, is in the Livrustkammaren Museum in Stockholm. This is the time most historians recognize as the beginning of the modern air gun.

In the 17th century, air guns, in calibers .30–.51, were used to hunt big game deer and wild boar. These air rifles were charged using a pump to fill an air reservoir and gave velocities from 650 to 1,000 feet per second (200–300 m/s). They were also used in warfare; the most recognized example being the Girandoni Military Repeating Air Rifle.

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 fired with greater rapidity than muzzle-loading guns.^[1] Moreover, they were quieter than a firearm of similar caliber, had no muzzle flash, and were smokeless. Thus, they did not disclose the shooter's position or obscure the shooter's view, unlike the Black powder muskets of the 18th and 19th centuries.

In the hands of skilled soldiers, they gave the military a distinct advantage. 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^[2] by the Tyrolean 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," "Giradoni"^[3] or "Girardoni".^[4]) The Windbüchse was about 4 ft (1.2 m) long and weighed 10 pounds (4.5 kg), about the same size and mass as a conventional musket. The air reservoir was a removable, club-shaped, butt. The Windbüchse carried twenty-two .51 caliber (13 mm) lead balls in a tubular magazine. A skilled shooter could fire off one magazine in about thirty seconds. A shot from this air gun could penetrate an inch thick wooden board at a hundred paces, an effect roughly equal to that of a modern 9 mm or .45 ACP caliber pistol.

Circa 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.

The celebrated Lewis and Clark Expedition (1804) carried a reservoir air gun. It held 22 .46 caliber round balls in a tubular magazine mounted on the side of the barrel. The butt served as the air reservoir and had a working pressure of 800 psi (5,500 kPa). The rifle was said to be capable of 22 aimed shots per minute and had a rifled bore of 0.452 in (11.5 mm) and a groove diameter 0.462 in (11.7 mm).

One of the first commercially successful and mass-produced air guns was manufactured by the W.F. Markham Co. Their first model air gun was called the Challenger and marketed in 1888. Their next model was the Chicago followed by the King. The Chicago model was sold by Sears, Roebuck for 73 cents in its catalog. In 1928 the name of the company was changed to King Mfg. Co. and remained so until the company was purchased by the Daisy air gun company.^[5]

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 in 1899, the National Smallbore Rifle Association was created. During this time over 4,000 air rifle clubs and associations existed across Great Britain, many of them in Birmingham. During this time, the air gun was associated with poaching because it could deliver a shot without a significant report.

Use

Air guns are used for hunting, pest control, recreational shooting (commonly known as plinking), and competitive sports, such as the Olympic 10 m Air Rifle and 10 m Air Pistol events. Field Target (FT) is a competitive form of target shooting in which the targets are knock-down metal silhouettes of animals, with a 'kill zone' cut out of the steel plate. Hunter Field Target (HFT) is a variation, using identical equipment, but with differing rules. The distances FT and HFT competitions are shot at range between 7.3 and 41.1 metres (24 and 135 ft) for HFT & 7.3 and 50.29 metres (24.0 and 165.0 ft)for FT, with varying sizes of 'reducers' being used to increase or decrease the size of the kill zone. In the UK, competition power limits are set at the legal maximum for an unlicensed air rifle, i.e. 12 ft·lbf (16 J).

Legal issues

While in some countries air guns are not subject to any specific regulation, in most there are laws, which differ widely. Each jurisdiction has its own definition of an air gun; and regulations may vary for weapons of different bore, muzzle energy or velocity, or material of ammunition, with guns designed to fire metal pellets often more tightly controlled than airsoft weapons. There may be minimum ages for possession, and sales of both air guns and ammunition may be restricted. Some areas require permits and background checks similar to those required for firearms proper.

Air gun power sources

The different methods of powering an air gun can be broadly divided into 3 groups: spring-piston, pneumatic, and CO₂. These methods are used in both air rifles and air pistols.^[6]

Spring-piston

Spring-piston air guns are able to achieve muzzle velocities near or greater than the speed of sound from a single stroke of a cocking lever or the barrel itself. The effort required for the cocking stroke is usually related to the power of the gun, with higher muzzle velocities requiring greater 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 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 and then cools as the air expands.

Spring-piston guns have a practical upper limit of 1250 ft/s (380 m/s) for .177 cal (4.5 mm) pellets. Higher velocities cause unstable pellet flight and loss of accuracy. This is due to the shock wave generated as the supersonic pellet contacts the air. Shortly after leaving the barrel, the pellet falls below the speed of sound and the shock wave overtakes the pellet, causing it to tumble. 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. Sonic crack from the pellet as it moves with supersonic speed also makes the shot louder sometimes making it possible to be mistaken for firearm discharge (unless the gun is silenced). Many shooters have found that velocities in the 800–900 ft/s (240–270 m/s) 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 midpoint (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 piston guns, especially high-powered ones, recoil as a result of the forward motion of the piston. Although the recoil is less than that of some cartridge firearms, it can make the gun difficult to shoot accurately as the recoil forces are in effect whilst the pellet is still traveling down the barrel. Spring gun recoil has a sharp forward movement too, caused by the piston hitting the forward end of the chamber when the spring has fully expanded. These two reactions are known to damage telescopic sights not rated for spring gun use.

Spring guns can also suffer from spring vibrations that reduce accuracy. These vibrations can be controlled by adding features like close-fitting spring guides or by aftermarket tuning done by "air-gunsmiths" who specialize in air gun modifications. A common modification is the addition of viscous silicone grease to the spring, which both lubricates it and dampens 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, their trajectory is consistent. Most Olympic air gun matches 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 competition. Today, the guns used at the highest levels of competition are powered by compressed air.

Gas Spring

Some makes of air rifle incorporate a gas spring instead of a mechanical spring. Pressurized air or nitrogen is held in a 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 units 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 long-term spring fatigue, lower recoil and faster "lock time" (the time between pulling the trigger and the pellet being discharged). The improvement in lock time results in better accuracy.

Pneumatic

Pneumatic air guns utilize 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 a reservoir. Pre-charged pneumatic guns' reservoirs are filled using either a high-pressure hand pump or by decanting air from a diving cylinder. This design, having no significant movement of heavy mechanical parts during the firing cycle, produces lower recoil.

Multi-stroke

Multi-stroke pneumatic air guns require the pumping 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.

Single-stroke

A single motion of the cocking lever is all that is required to compress the air. 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. Single-stroke pneumatic rifles dominated the national and international ISSF 10 metre air rifle shooting events from the 1970s up to the 1990s.

Pre-charged pneumatic (PCP)

Pre-charged pneumatic (PCP) air guns 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 lower operating costs compared to CO₂ guns. PCP guns have very low recoil and can fire as many as 500 shots per charge. The ready supply of air has allowed the development of semi and fully automatic^[7][8] air guns. PCP guns are very popular in the UK and Europe because of their accuracy and ease of use. They are widely utilized in ISSF 10 metre air pistol and rifle shooting events, the sport of Field Target shooting,^[9] and are usually fitted with telescopic sights.

Early hand pump designs encountered problems of fatigue (both human and mechanical), temperature warping, and condensation. None of which are beneficial to accurate shooting or air gun longevity. Modern hand pumps have built-in air filtration systems and have overcome many of these problems. Using scuba-quality air decanted from a scuba cylinder provides consistently clean, dry, high-pressure air.

Unregulated action

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 decreases with each successive shot. As a result, when the reservoir pressure is at its peak, 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 self-regulation that gives a greater consistency of velocity from shot to shot. A well-designed PCP will display good shot to shot consistency as the air reservoir is depleted.

Regulated action

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.

CO₂

Most CO₂ guns use a disposable cylinder, a powerlet, that is purchased often pre-filled with 12 grams of pressurized 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. So, as long as the CO₂ cylinder is not allowed to cool significantly (through firing), the pressure will not decrease by a significant degree. However, at higher temperatures (e.g. on a hot summer day), this approximation does not hold true.

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 standardized by Crosman.

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 120 °F (49 °C) 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.^[10]
• Cooling Each time the gun is fired there is rapid 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.

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 (272–408 m/s ~ 893–1340 ft/s). Pellets are also manufactured from tin, or a combination of materials such as steel-tipped plastic.

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 4.5 mm/.177" diameter. Lead "Round Balls" are manufactured in numerous calibers too; 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. Lead number 3 buckshot pellets can be used in .25" caliber airguns as if they were large BBs.

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 is discouraged.

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).

Darts and arrows

Air guns can also fire darts or arrows using a similar mechanism to a pneumatic nail gun.^[11] This type of air gun is usually homebuilt and typically uses various high-pressure tanks as its power source, ranging from oxygen tanks to fire extinguisher. Only Smoothbore barrels are recommended for air dart ammunition. These are usually limited to .177 cal.

Calibers

The most common air gun calibers are

• .177 (4.5 mm): the most common caliber. All official shooting organizations mandate .177 caliber for both pistol and rifle competition. 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. Can be effectively used to hunt small quarry or vermin.^{[citation needed]}
• .22 (5.5 mm & 5.6 mm): for hunting.

Other less common traditional calibers include:

• .20 (5 mm): initially proprietary to the Sheridan multi-pump pneumatic air rifle, later more widely used.
• .25 (6.35 mm): the largest commonly available caliber for most of the 20th century.^{[citation needed]}

Larger caliber air rifles suitable for hunting large animals are offered by major manufacturers. These are usually PCP guns. The major calibers available are:

• .357
• .45 (11.43 mm)
• .50 (12.7 mm)^[12]
• .58 (14.5 mm)

Custom air guns are available in even larger calibers such as 20 mm (0.79") or .87 (22.1 mm).^{[citation needed]}

See also

• Air gun laws
• BB gun
• Blowgun
• Field Target
• Girandoni Air Rifle
• Gun safety
• Gun Quarter
• Hunter Field Target
• List of air guns
• Marksmanship
• Pellet

References

External links

Wikimedia Commons has media related to Air guns.

• The Airgun Society
• Robert Beeman's History of Airguns
• Modern Airgun and Air Rifle Technology, & Sport
• Trev's Airgun Scrapbook http://www.cinedux.com