barrier formed by wall portions 150 and 152 and initiate the facing shaped charge 144 in second explosive device 132 the explosive transfer occurs through second explosive device 132 in reverse order from that just described for first explosive device 130 resulting in the configuration seen in fig3 eventually a firing device in the first gun 52 attached to lower housing portion 76 is initiated this sequence is repeated through the other gun sets 50 and bi directional explosive transfer subassemblies 56 eventually firing the lowermost second gun 54 assuming that there is no break in the firing sequence there may be occasions when it will be desirable to initiate gun string 44 from the far end in this event a firing head such as time domain firer 58 is fired which initiates the firing of the lowermost second gun 54 which in turn triggers the lowermost first gun 52 to fire the lowermost first gun 52 initiates second explosive device 132 in the lowermost bi directional explosive transfer subassembly 56 the explosive transfer in this case follows an upward path through bi directional explosive transfer subassembly 56 to detonate the next gun set 56 this sequence is repeated upwardly until the uppermost gun.