rings 40 and 42 below opening 38 thus in the position shown in fig1 d the opening or openings 38 are effectively sealed against the retaining sleeve 14 openings 38 lead to passage 44 which is in fluid communication with a breakable member such as a rupture disc 46 34 rupture disc 34 is defined broadly as any device that prevents flow and upon certain conditions permits flow so that it includes members that break dissolve or move so that a valve member is also within the definition a hammer 48 is sealingly retained in passage 44 by shear pin 50 those skilled in the art will appreciate that actuating of the gun g occurs when the rupture disc 46 breaks and hydrostatic pressure in the wellbore acts on hammer 48 driving it down to break the shear pin 50 to set off the gun g accordingly the details of the mechanism for shooting the gun g beyond stating that it is hammer actuated will not be explained in detail because it is known to those of ordinary skill in this art the time it takes to break the rupture disc 46 can be varied by sizing the opening or openings 38 to restrict flow or by adding a restriction such as an orifice 39 the apparatus a further includes a piston 52 disposed between an outer tubular extension 54 of the upper body 20 of gun g the outer tubular extension 54 is attached at thread 56 an inner tubular extension 58 extends from the upper body 20 and defines ports 60 which allow fluid communication from an internal passage 62 to an annular passage 64 where the piston 52 is disposed piston 52.