Abstract: A firing head assembly has a sealed chamber containing a piston, a firing pin, and an impact detonator. The firing head assembly and a perforating charge are installed within a sub and the sub is secured into a string of conduit being lowered into a wellbore. After cementing the conduit, the operator drills out the cement in the conduit, disintegrating the chamber and exposing the sealed chamber to the fluid pressure of the drilling fluid in the conduit. The drilling fluid pressure causes the piston to drive the firing pin against the detonator, which detonates the perforating charge. The operator then pumps down a logging tool to survey the well. Fluid in the conduit below the pump-down head can flow out the displacement perforation into the earth formation while the logging tool is moving downward.