A model 1911 semi automatic pistol having a receiver, a barrel, a breach slide and a firing mechanism. The barrel is coupled to the receiver. The breach slide coupled to the receiver. The firing mechanism is coupled to the receiver. The firing mechanism is adapted for double action operation. The firing mechanism has a hammer with a firing pin strike surface and an engagement surface enabling single action operation of the firing mechanism. The engagement surface is movable relative to the firing pin strike surface.