What is a gas piston (gas ram, nitro piston) and how is it different?

A gas piston replaces the coiled steel spring with a sealed cylinder of compressed gas, usually nitrogen.

Cocking the gun further compresses that gas; releasing the trigger lets it expand and drive the piston forward. Compared to a coil spring, a gas piston cocks more linearly, has less vibration and twang, can be left cocked for hours without spring fatigue, and is less affected by cold weather. Coil springs are cheaper, more easily tuned by hobbyists, and replaceable; gas rams are usually replaced as a unit if they fail. Accuracy potential is similar.