combustion chamber where it contacts the hot compressed gas therein and ignites as soon as this liquid propellant begins to burn in the confined combustion chamber the pressure of the gas in the combustion chamber will rise when the force developed by the gas pressure forward of the piston head exceeds the force developed by liquid pressure aft of the piston head regenerative action begins the rings of the piston head open and the piston head and the check valve head move aft initially closing the check valve the charge of liquid propellant trapped forward of the check valve is progressively injected through the open rings of the differential piston head into the combustion chamber as the differential piston head is forced aft by the combustion gas pressure the holes 80 through which the air originally entered will not be exposed and opened until the differential piston has almost completed its aftward stroke the high temperature combustion gas is passed out of the combustion chamber to ignite the main charge of liquid propellant in the gun when the combustion gas pressure becomes sufficiently high to compress the bellevile washers 24 to unseat the relief cap 16 to allow the combustion gas to flow out through the holes 82 this occurs before the completion of the differential piston stroke and as the differential piston continues its aftward movement a sustained pulse of ignition gas is passed out through the holes 82 the fine bore 39 and the groove 40 provide a vent to allow leakage to be relieved to a passageway 100 in the housing 10 of the gun which opens to atmospheric pressure this vent also serves to provide the differences in area between the front and the rear faces of the differential piston head 1 no pyrotechnic.