Air guns are weapons that propel a bullet using compressed air. They are also called pneumatic guns.

There are three completely different types of air gun: spring-piston, precharged cylinder, and reservoir.

Table of contents
1 Spring piston guns
2 Cylinder guns
3 Reservoir guns

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." The piston and cylinder of a spring-piston air gun must be lubricated with a special oil that will not burn at these high temperatures.

Most spring piston guns are single shot breech loaders by nature (somewhat like an old shotgun) but multiple-shot weapons 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.

Spring air guns 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 thorugh the 1970s and into the 1980s being short with spring-piston guns. Beginning in the 1980s, guns powered by compressed, liquified 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 (2000-3000psi)

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.

Since most spring-piston guns have muzzle velocities near the speed of sound, the normal bullet is a "pellet" shaped like two cones joined at the apex. This shape is optimally stable near the speed of sound- it does not tumble.

Most spring piston air guns have a calibre of .177, and are designed for target practice. Cost per round is less than $0.02 (US) for olympic-quality ammuntition, and far less for cheaper grades. .20 .22 and .25 calibre guns exist, and are mainly used for hunting and fieldtarget 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 tank (>300 bar).

Reservoir guns

Reservoir 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 resevoir guns in larger calibers (6mm to 9mm) 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), and could shoot through armor. In this era, France had a special detachment of snipers who carried air-rifles.