Abstract: A free piston machine comprising a cylinder having a first and second closed ends, a piston slidable in the cylinder between the closed ends, a first conduit communicating between a source of gas under pressure and the first closed end of the cylinder, a second conduit communicating between the first closed end of the cylinder and atmosphere, valve means controlling flow through the second conduit, and a piston-controlled port in the cylinder and communicating between the cylinder and the source of gas under pressure, the arrangement being such that the piston closes the port when disposed at the first closed end of the cylinder.

Abstract: A gunfire simulator has a combustion chamber, means for admitting fuel gas to the combustion chamber, inlet valve means for admitting oxygen or an oxygen containing gas to the combustion chamber, ignition means for igniting fuel gas in the combustion chamber, an exhaust port in the combustion chamber and outlet valve means for closing the exhaust port and arranged to open rapidly in response to a pressure rise within the combustion chamber. Preferably the outlet valve means comprises a frangible diaphragm, which may be clamped by its marginal portion in a breach block during combustion.