A review on the physiology of Insulin like Growth Factor-I (IGF-I) ... by Debnath, Surajit / Advances in Environmental Biology.
The spring piston airgun generates power using a powerful spring-loaded piston that is housed within a compression chamber. Cocking the gun (using a break barrel, side lever, or under lever mechanism) causes the piston assembly to compress the spring. When the spring is released it pushes the piston forward compressing a column of air in the chamber behind the pellet.
The power to drive the piston is supplied by an electric motor, internal-combustion engine, or other power source - although more primitive piston pumps may be driven by hand, wind, or flowing water. The motion behind the piston is usually rotational.