A removable gas supply for a weapon simulator, wherein the gas system cycles the bolt or slide of the firearm to provide recoil, actuate a hammer/trigger mechanism and provide realism of the original weapon functions. The weapon simulator has a housing defining a piston chamber and a piston positioned in said chamber and connected to the bolt. The simulator also includes a valve chamber in the housing connected with said regulated gas supply and said bolt, and a recoil valve positioned in the valve chamber to control the release of gas from said regulated gas supply to said piston chamber to generate recoil in the weapon simulator.