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Air pistol

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.

There are many different types of air guns: spring-piston, multi-pump pneumatic, precharged pneumatic (PCP), and reservoir.

Pneumatic rifles have been around since the 15th century. It has been well documented that Austrian soldiers sniped and killed French soldiers under Napoleon Bonaparte with them.

Spring piston guns

Spring-piston air guns achieve muzzle velocities near the speed of sound from a single, not-too-difficult cock.

Cocking a spring-piston gun compresses a large steel spring inside the gun. The trigger releases the spring. The spring moves a piston, which instantaneously compresses the air in a chamber. The hot (often exceeding 1000°C) air moves the pellet down the bore of the gun. There are no adiabatic losses because the air does not have time to cool. This high efficiency is why the gun is so easy to "pump." Usually a molybdenum disulfide based grease is used due to it's excellent high pressure lubrication properties and mild burning rate (moderate to high power spring guns often obtain 30% or more of their energy from combustion of trace amounts of lubricant. Using a lubricant that is too volatile can result in detonation/dieseling and damage. At one time silicon based chamber lubes were recommended but their poor performance under high pressure has lead to their general disuse.) Spring-piston guns seem to have a practical upper limit of 1200 ft/s (370 m/s) for .177 cal (4.5 mm) pellets.

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 short 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 lb/in&sup2 (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.

The typical projectile used in rifled airguns is the lead diablo pellet. This is a wasp-wasted projectile open at the base and having a variety of head styles. The diablo 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 it that the high drag produces short maximum ranges which adds to safety. The diablo 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 spring piston 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. 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.

A gas gun has also been used in space on EVAs for moving around, e.g. the Hand-Held Maneuvering Unit (HHMU), using compressed oxygen, and the Crew Propulsive Device (CPD), using compressed nitrogen.

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 mm to 9 mm) are often used for hunting small game.

In the days of Louis XIV of France, reservoir guns were truly fearsome, as they could be fired in all weather (unlike flintlocks, which were very susceptible to rain), and could shoot much faster than the slow, muzzle-loading guns of the period. Moreover, they were nearly silent (especially in the middle of a battle!), had no muzzle flash and did not produce a telltale cloud of smoke. In this era, France, Austria and other nations had special detachments of snipers who carried air-rifles. These airguns were no real challenge to the dominant position of powder weapons. They were expensive, delicate and not very powerful. The pumped-up air reservoirs could burst explosively. The valves were not really tight and slowly lost pressure.

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' long and weighted 10 pounds, which was about the same size and mass as a conventional musket of the time. It carried the reservoir in a removable, club-shaped butt. The troopers carried extra butts pumped up. The Windbüchse carried twenty lead balls of caliber 13mm (.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 9mm or .45 inch pistol. A shot from this gun could penetrate a one-inch wooden board at a 100 paces.

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). 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.

Airgun Artillery

The USS Vesuvius was a ship that carried three 15-inch "pneumatic guns", effectively cannon-sized reservoir airguns.