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.
4.13 Air in a spring loaded piston/cylinder has a pressure that is linear with volume, P. = A + BV. With an initial state of P = 150 kPa, V = 1 L and a final state of 800. kPa and volume 1.5 L it is similar to the setup in Problem 3.16.