A weapon, representatively a machine gun, is mounted in a crew-served orientation on the outer end of an elongated support beam extending transversely through and secured within the cabin area of a helicopter, with the outer beam end disposed exteriorly of the helicopter. A seat structure, upon which a crew member may sit, is mounted atop a longitudinally central portion of the beam for sliding movement along the beam toward and away from the machine gun. Operatively associated with the machine gun is an ammunition box having a lid portion fixed to the bottom side of the beam end upon the top side of which the machine gun is pivotally and rotationally mounted. The body of the ammunition box is removably securable to the beam secured lid.