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.
A piston can also shatter if an engine sucks a valve or the head breaks off of a valve. A dropped valve can occur if the valve spring keeper fails or pulls through (sometimes as a result of over-revving the engine), or the valve spring fails and allows the keeper to pop loose.