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.
Most airguns generate relatively low power compared to a rimfire. This is not always the case; there are a couple pre-charged pneumatic airguns in my collection that are getting close to 600 fpe. However, most (not all) spring piston guns are limited to under 20 fpe.