Perforation guns comprise a firing head to activate an initiator which, in turn, causes detonation of perforation charges. The firing head comprises a firing pin in sliding engagement with a bore of a housing. An opening in the housing places the firing pin in fluid communication with a wellbore annulus when the perforation gun is disposed in the wellbore. A release mechanism maintains the firing pin in it is initial position. Upon being disposed in the desired location within the wellbore, the release mechanism is actuated allowing the firing pin to move within the bore. Hydrostatic pressure acts on the firing pin in a first direction that is determined by a bias provided by one end of the firing pin providing greater resistance to movement as compared to the other end. Movement of the firing pin causes the firing pin to strike the initiator causing detonation of the perforation charges.