may be removed as a unit the auxiliary rail member 40 may include a high precision rail clamp which allows it to be removed and replaced without the need to re boresight the fire control system c and scope e to the weapon 10 the auxiliary clamp may employ for example a three point clamp of the type described in commonly owned u s pat no 7 685 759 the entire contents of which are incorporated herein by reference a handgrip unit d is attached to the rail system 12 and extends substantially vertically downward from a forearm portion of the weapon 10 the handgrip unit d includes a rail grabber 50 at its upper end and a generally hollow handgrip housing portion 52 in certain embodiments the handgrip housing 52 may receive a retractable bipod assembly 64 which may be for example as described in commonly owned u s pat nos 7 841 120 and 7 712 241 each of which is incorporated herein by reference in its entirety in alternative embodiments the handgrip housing may house an auxiliary rechargeable battery or battery pack 56 for the smartphone b wherein the auxiliary battery 66 is electrically coupled to the smartphone via the circuit of the rail system 12 in alternative embodiments the auxiliary battery may also be used to power other weapon mounted accessories including the ranger finder and the fire control system in still further embodiments the bipod assembly 64 and the auxiliary battery battery.