A hydraulic safety pin is interconnected between a device, such as a pressure controlled firing head associated with a perforating gun, and a small diameter tubing in a wellbore. When, the small diameter tubing is filled with fluid under pressure, the hydraulic safety pin allows the fluid to fill the small diameter tubing but prevents the pressure of the fluid in the small diameter tubing from accidentally detonating the firing head when the pin is disposed in a first position; however, when the tubing is already filled with fluid under pressure, the hydraulic safety pin allows the pressure of the fluid in the small diameter tubing to detonate the firing head when the pin is disposed in a second position. The hydraulic safety pin may be installed in a special coupling, and multiple special couplings may be used in a tool string disposed in a wellbore.