in a tubing conveyed perforating gun assembly having shaped charges to perforate a well , an improved pressure sequence shaped charge detonator is included . the preferred form of device is appended to the bottom of the assembly to function as a backup detonator . first and second piston and cylinder assemblies in an elongate body provide pressure initiated arming and subsequent pressure initiated firing pin movement to achieve primer cord detonation for perforation of the well . the pressure sequence is controllable , selectively to a specified pressure to arm and thereafter to fire .