Air gun

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

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. 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 is why the gun is so efficient.

Spring piston guns usually cock by pulling down on the barrel so that it hinges near hand grip. Most are single shot breech loaders by nature (somewhat like an old shotgun).

These have long service lives. Because they deliver the same energy on each shot, the trajectory is extremely repeatable. The air-pistol used by most Olympic air-pistol competitors is a spring-piston gun, a "Feinwerkbau 65."

The chinese army uses spring piston small arms to training more economically. Surplus military-issue chinese spring-piston air-guns are sometimes widely available by mail-order.

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

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).

Reervoir 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 historical air-guns were reservoir guns. The air gun carried by Lewis and Clark was a reservoir gun.

Some 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.

In the days of Louis XIV of France, reservoir guns were truly fearsome, since they shot in all weather (unlike flintlocks), and could shoot through armor. In this era, France had a special detachment of snipers who carried air-rifles.