A simple to manufacture high pressure rechargeable ammunition cartridge comprising a hollow pressure casing (20) providing a gas storage space (19) to hold compressed gas therein, with a discarge passage (25) in a front part of said casing said gas storage space; an actuation passage (24) extending into said casing from the rear; a valve opening (52) at a front end of said actuation passage, and a valve member which has a normal position wherein it closes said valve opening and blocks said discharge passage, and is movable bodily forwards as shown from said normal position to open both said valve opening and said discharge passage to allow gas to escape from the pressure casing directly through both of said passages to drive a missile. The valve member may be modified so as to be dischargeable from the cartridge and may form part of a missile.