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.
Some of the current production guns I think are best in class for spring piston hunting arms include the HW 95, HW 80, RWS 48, RWS 350 and the AirArms TX200, along with my favorites older guns the Beeman/Webley C1 .177 and the Webley Patriot .25.