Abstract: A perforating gun is suspended downhole in a cased borehole, in underlying relationship relative to a packer device, and adjacent to a hydrocarbon-bearing formation to be completed. A connecting tubing interconnects the gun with the packer device, and includes a rupture barrier therebetween which prevents debris from accumulating within a gun firing head. An upper tubing string is removably connected to the packer device and forms a passageway which extends from the surface of the earth, down through the packer, and to the gun firing head. The upper tubing string includes a seal means and an extension tube which sealingly engages the packer device and penetrates the rupture barrier to provide a passageway through which the gun can be fired from the surface of the earth. This combination of elements prevents malfunction of the gun when the tool is left downhole for an appreciable length of time.