sidewall of the forward housing portion 36 overlaying respectively the forward and rear housing openings 68 70 the forward groove 104 cooperates with the inner surfaces of the casing sidewall 16 to define a forward annular chamber not numbered communicating with the forward opening 68 a pair of annular grooves 108 110 one to either side of the forward groove 104 carry o rings 112 114 that seal that chamber another pair of annular grooves 116 118 carrying o rings 120 122 similarly isolate the rear groove 106 and a rear chamber not numbered effectively defined by the rear groove 106 and internal casing surfaces the gas delivery path 48 includes a path segment 124 internal to the casing sidewall 16 that communicates with the forward annular groove 104 the gas delivery discharge path 50 is a conduit formed internally to the casing sidewall 16 and places the rear annular groove 106 in communication with the discharge valve 28 the coupling of the discharge flow path 50 to the discharge valve 28 involves a groove and o ring arrangement similar to that described above the flow paths internal to the casing 14 can be formed by longitudinal and lateral drilling and appropriate plugging of open ends the operation of the gun 10 will be described from prior to installation of the gas cartridge 12 the expandable rear chamber 64 is contracted under the influence of the external coil spring 92 the forward projection 86 of the piston assembly 76 has displaced the valve ball 72 to its.