Air gun
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Air guns are weapons that propel a bullet using compressed air or another gas, possibly liquefied. They are also called pneumatic guns, pellet guns, or BB guns.
Modern air guns are typically low-powered for safety concerns, but high-powered designs have been used for hunting and military applications. They do benefit from very high accuracy, and are used in sharpshooting events in the Olympic Games.
Most airguns can be practiced in a backyard or garden, and even indoors with the proper backstop. Some of the stronger power "springers" can propel a pellet beyond 1100 ft/s (340 m/s) at approximately the speed of sound. It will produce a noise similar to a .22 cal (~5.6 mm) rimfire. These pneumatic rifles can be found in the following calibers, .177 (~4.5 mm) most common, .20 (~5.1 mm) Benjamin Sheridan, .22 (~5.6 mm) most common for hunting, .25 (~6.4 mm) and even 9 mm.
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Powerplant technology
There are many different types of air guns in terms of powerplants that are used to get the air moving: spring-piston, multi-pump (multi-stroke) pneumatic, single-stroke pneumatic, precharged pneumatic (PCP), and reservoir. All of them have advantages and disadvantages.
Multi-stroke pneumatic airguns
They require 2-10 pump-ups of an on-board lever to store compressed air in the powerplant.
Advantages: virtually recoilless, completely self-contained, the pellet velocity of 300 to 800 ft/s (90 to 240 m/s) can be adjusted with different numbers of pumping strokes.
Disadvantages: You must pump your gun each time you want to fire.
Single-stroke pneumatic airguns
They only require one pump to fully charge the gun. As a result: it is easy to cock, highly consistent and accurate. However, the power of one single stroke is usually low, e.g. .177 pellets at 500 to 600 ft/s (150 to 180 m/s).
Spring-piston guns
Spring-piston air guns achieve muzzle velocities near the speed of sound from a single, not-too-difficult cock.
Spring-piston guns operate by means of a spring-loaded piston in a chamber separate from the barrel. Cocking the gun compresses the spring; pulling the trigger releases it and causes the piston to drive air into the barrel. The spring is usually made in the form of a steel coil.
As the air compresses, it becomes very hot (Charles' Law), often in excess of 1000 °C. Because of the rapidity of the firing process, very little of this heat is lost into the gun's metal parts. Consequently, spring-piston guns are more efficient than reservoir guns. Lubricants (such as molybdenum disulfide) are generally designed so that they burn gently at this temperature; upwards of 30% of the energy of the shot may come from this effect. On the other hand, excessively flammable lubricants may detonate and damage the gun.
Spring-piston guns seem to have a practical upper limit of 1200 ft/s (370 m/s) for .177 cal (4.5 mm) pellets.
Break-barrel.gif
Barrel cocking"
Most spring piston guns are single shot breech loaders by nature (somewhat like an old shotgun) but multiple-shot guns have been increasingly common in recent years. Spring guns are typically cocked by a mechanism is which the gun is hinged at the mid-point, with the barrel serving as a cocking lever. Other systems used include side levers, under-barrel levers and motorized cocking, powered by a rechargeable battery.
Unlike cartridge firearms, the spring is very powerful in these guns and is held back by a sear that has a very small engagement area. There have been cases of severe crushing and even amputation when the spring has been released unexpectedly. Always use one hand to restrain the cocking lever when loading these guns so that should the sear slip, you will not be injured.
Spring guns, especially the higher powered ones, have a tendency vibrate/recoil quite a bit. Although this recoil is not comparable in magnitude to that of a cartridge firearm, it can make the gun difficult to shoot accurately as the vibration is well under way while the pellet is still in the barrel. Most guns seem to respond well to a light repeatable hold that allows the gun to vibrate the same way from shot to shot. Spring gun recoil also has a sharp forward component that is well known for breaking telescopic sights. On any but the lowest power spring guns, any telescope should be airgun rated. Spring gun tuning can reduce vibration to very low levels. Airgunsmiths specialize in airgun modifications.
The better quality spring air guns can have long service lives, often exceeding thirty years. Because they deliver the same energy on each shot, the trajectory is extremely repeatable. This repeatability resulted in most Olympic air gun matches through the 1970s and into the 1980s being shot with spring-piston guns. 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 lbf/in² (14 to 21 MPa).
The Chinese army uses spring-piston small arms to train more economically. Surplus military-issue Chinese spring-piston air-guns are sometimes widely available by mail-order.
Gas Piston Guns
Some more expensive models use a "gas piston"--pressurized air is held in a special chamber built into the piston, and this air is pressurized when the piston is cocked. Gas piston guns require higher precision to build, since they require low friction sliding seal that can withstand the high pressures when cocked. The advantages of gas pistons include the ability to vary the power of the gun by changing the amount of air in the gas piston, the reduction in moving mass during firing (and a corresponding reduction in felt "recoil" caused by the piston and spring starting and stopping), and elimination of the problems of spring fatigue. A notable disadvantage should be the loss of energy as dissipated heat.
One specialized adaptation is the light gas gun, which usually uses a gunpowder propelled plastic piston compressing a cylinder full of hydrogen gas. Light gas guns are capable of propelling a 5 mm projectile at speeds of up to 8000 m/s.
Ammunition
The typical projectile used in rifled airguns is the lead diabolo pellet. This is a wasp-waisted projectile open at the base and having a variety of head styles. The diabolo pellet is designed to be drag stabilized. This, in addition to the spin afforded by the rifling, makes the airgun one of the most accurate of all guns. Another advantage of the diablo pellet is that the high drag produces short maximum ranges which adds to safety. The diabolo pellet is not a stable as some other shapes in the transonic region. While some high power spring guns can propel light pellets at or beyond the speed of sound, this results in decreased accuracy and often decreased life of the spring and seals (the low momentum of the light pellet causing it to start moving down the barrel before maximum pressure is reached resulting in loss of the air cushion and subsequent slamming of the piston/seal into the end of the chamber at high velocity.)
Most air guns have a calibre of .177 (4.5 mm), and are designed for target practice. Cost per round is less than $0.02 (US) for Olympic-quality ammunition, and far less for cheaper grades. .20 .22 and .25 calibre (~5.1, ~5.6 and ~6.4 mm) guns exist, and are used mainly for hunting and field target shooting.
Cylinder guns
Cylinder guns shoot from a purchased cylinder, usually filled with liquefied carbon dioxide. Most paintball guns are this type. There are also CO2 charged airguns that are very popular for plinking.
Precharged pneumatic airguns
PCP (pre-charged pneumatic) airguns can be used for hunting and competition. These are usually filled from an air reservoir, such as a diving cylinder.
See also Airsoft.
Reservoir guns
Reservoir guns, sometimes called "multi-pump" guns, have a pump to compress air into a reservoir. The air cools, losing much of the energy. These are neither fun nor fast. If a single shot needs more than one pump, it's probably a reservoir gun.
Most reservoir guns use the same ammunition as spring-piston guns.
Most historical air-guns were reservoir guns. The air gun carried by Lewis and Clark was a reservoir gun.
Many reservoir guns have been used for hunting. One of the traditional weapons for hunting wolves in Russia was said to be a large-calibre reservoir air-rifle. It is said to have shot silently to avoid warning the pack. Modern reservoir guns in larger calibers (6 to 9 mm) are often used for hunting small game.
The multi-pump and CO2 pneumatics have been popular in the United States, where they are known as "BB guns" or "pellet guns," depending upon the type of projectile used. These are typically viewed in other countries as children's toys. There are exceptions to this, as companies such as Benjamin Sheridan, Crosman, and Daisy market sophisticated systems (though Daisy and Crosman, also manufacture children's guns).
Choosing an airgun
- Self-contained: spring-piston, multi-stroke pneumatic, single-stroke pneumatic. These guns require no additional CO2 cylinders, and are thus cheaper to operate.
- Noise Level: spring-piston. Absence of loud gas discharges makes these guns quieter to operate. Consider when practicing in cramped urban areas.
- Accuracy: pre-charged, single-stroke pneumatic. Without the variable factor introduced by CO2 cylinders 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.
The above points are generalizations. The overall performance of your airgun will depend on its quality. For instance, a match-grade CO2 rifle will have better accuracy than a cheaper spring-piston import. The extra cost translates into greater quality, accuracy, shot-placement, etc.
When choosing an air rifle or an air pistol consider where (club range, backyard, condo, farm) and how (competition, target practice, plinking, pest-control) you plan to use it.
History
Reservoir guns represent the oldest pneumatic technology; they have existed since the 15th century. They presented some compelling advantages over the primitive firearms of the day. These airguns could be fired in wet weather (unlike flintlocks), and with greater rapidity than the muzzle-loading guns of the period. Moreover, they were nearly silent, had no muzzle flash, and did not produce telltale clouds of smoke. In this era, France, Austria and other nations had special detachments of snipers who carried air-rifles.
For general usage, though, airguns were no real challenge to the dominant position of powder weapons. They were expensive, delicate and not very powerful. The air reservoirs could burst explosively. The valves were not very tight and slowly leaked pressure. Historical accounts mention that the common soldiers were often unable to handle the complex, delicate guns, which was logical in an age where the average peasant recruit had never encountered a machine more complex than a horse-drawn cart, and people who had any experience with mechanical devices (millers or clockmakers) were few and far between. The guns of the period were famously crude, and required very little skill from the average infantryman (skilled snipers or hunters were something else entirely, see Jäger).
The Austrian 1780 model was named "Windbüchse" (literally "wind rifle") in German. The guns were developed by the gunsmith Bartholomeo Girandoni (1744-1799), and are occasionally called "Girandoni air guns" in literature. 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 of the time. The reservoir was a removable, club-shaped butt; the gun was recharged by replacing the exhausted reservoir with a new one. The Windbüchse carried twenty lead balls of caliber 13 mm (.52 inch) in a tubular magazine, and could empty one magazine in about thirty seconds. The effect can be roughly compared to that of a modern 9 mm or .45 inch pistol. A shot from this gun could penetrate a one-inch wooden board at a 100 paces.
The celebrated expedition headed by Lewis and Clark reportedly carried a large-caliber reservoir gun.
Some of the largest pneumatic guns in history were mounted on the USS Vesuvius. This naval vessel was armed with three 15 inch (381 mm) reservoir-powered cannons that fired highly explosive projectiles. Unfortunately for the future of airguns, these suffered from poor range and, unlike conventional cannon, could not rotate relative to the rest of the ship.