is employed to initiate the detonation of the prima cord arm 130 is pivotally connected at 132 to the gun sub and at 134 to extension lever 136 which extends the arm upon energization of the gun sub by means of the electrical conductor contained within the wire line numeral 138 is a recess within which the arm is retractably accommodated shaped charges 140 and 144 are aligned parallel to one another and connected to the prima cord which extends through the chamber forming bulkheads 143 for example each of the bulkheads contain an explosive responsive valve device 164 the details of which are set forth in fig5 and 6 the valve device 164 includes spaced passageways 160 39 160 which lead into and out of a chamber containing balls 166 of various different diameters the ball containing chamber is funnel shaped with the small balls being located in the upper cylindrical portion of the chamber while the large balls are located in the lower converging part of the chamber in fig4 the spaced charges form spaced tunnels 192 and 194 which extend parallel to one another horizontally away from the gun so that communication can be established therebetween at 190 as a result of the pressure differential produced by chamber 42 in operation and in order to practice the method of the first embodiment of the present invention.