Abstract: A perforating gun can be conveyed on tubing and fired while disconnected from the tubing. The gun is retained to a running tool at the lower end of the tubing in a locked position. Actuating a plunger by pressure, a weight bar or other techniques, breaks a rupture disc and allows use of hydrostatic pressure to stroke a piston and defeat the lock between the gun and the running tool. Upon exposure of ports on the gun to wellbore hydrostatic due to movement out of the running tool, the firing sequence in the gun is initiated. The weight of the gun, as well as hydrostatic or applied pressure in the wellbore, drives the perforating gun out of the running tool.