How does an airsoft gun work?

How does an airsoft gun work?
Air soft guns are recreational guns that shoot small pellets, or bb's, at a high speed, but not high enough to kill. Air soft guns are designed for sport and entertainment. They are designed to be shot at people in recreational games. The three types of air soft guns are gas-powered guns, spring-powered guns and automatic electric guns.

Gas-Powered Guns
Gas powered guns operate much like an air bb or pellet gun. The gun is powered by a compressed form of propane gas, called 134a green gas. The compressed gas is loaded into the gun and a portion of the gas is released at high pressure. The high pressure is targeted at the pellet or bb in the gun, pressure then causes the pellet or bb to accelerate out of the gun. The gas source must be replaced frequently.

Spring-Powered Guns
The spring-powered gun uses a tension spring to impart velocity on the pellet or bb. The spring is cocked by manual power, meaning the person holding the gun provides the force necessary to put the spring in a tensed position. When you pull the trigger, the tension in the spring is released into the pellet or bb, which causes the pellet or bb to release from the gun at a high velocity. Because the spring must be manually cocked for each pellet or bb, the spring-powered gun is limited to single fire shooting.

Automatic Electric Guns
The automatic electric gun is the most expensive, and probably the most popular, form of air soft gun. These guns run on electric battery power. The battery powers a small motor which imparts acceleration onto each pellet or bb. Because the motor can run continuously, these guns can be automatic, or they can be restricted to single fire or semi-automatic. The batteries have to be replaced or recharged after high usage.

Source:
http://www.instructables.com/id/The-Ultimate-Guide-To-AirsoftingReally/step10/