fluid pressure chamber pc through the unidirectional check valves 9 provided in the fluid passages 7d and 7c and raises the fluid pressure in such fluid pressure chamber to the level existing in the casing annulus below the packer the tubing pressure may then be decreased by bleeding off the pressure a further decrease if desired can be obtained by swabbing operations in the tubing string in any event the fluid pressure in the casing annulus surrounding the housing assembly 2 will be substantially reduced below the level of the pressured fluid trapped within the fluid pressure chamber pc when the fluid pressure differential across the hammer 40 reaches a desired level the locking sleeve piston 46 is shifted by such fluid pressure differential shearing the shear pin 46c the hammer 40 can then cam the collet arms 45c outwardly and drive down into impact engagement with the primer 32 under the force of the differential fluid pressure the detonation of primer 32 effects the detonation of the booster charge 33 and in turn the ignition of the intermediate primer cord 34 which travels down the primer cord 34 to detonate the booster charge 35 detonation of booster charge 35 will effect the detonation of the booster charge 36 contained within the upper end of the perforating gun 20 thus effecting the detonation of the primer cord 26 extending through the perforating gun 20 to detonate each of the shaped charges 21 mounted therein another.