Cocking the gun moves a lever forward, causing the spring to compress behind the piston. As the piston moves to the rear, it allows air to enter the cylinder. The spring automatically locks into position until the gun is ready to be fired. When the trigger is pulled, the spring is released driving the piston forward compressing air in the cylinder. The compressed air then passes through a small hole into a chamber and the rear of the gun barrel where it expands, forcing the BB through the barrel to its target.
Pneumatic airguns have a pump lever which operates the air piston.
Pumping the lever allows air to enter behind the piston. Air passes through a valve which enables the gun to build pressure. This process allows compressed air to build in the chamber so that when the trigger is pulled, compressed air is released into the barrel and drives the projectile forward through the barrel and out to the target. The unique propulsion system of the CO 2 Method allows repeated firing without pumping or cocking the spring - something the other two methods cannot do. Charged CO 2 cartridges are locked into place in the handle of the gun.