An ammunition magazine box is provided for receiving a vertically serpentined length of belted ammunition for selective outfeed from the box to a machine gun. A specially designed support bracket structure, representatively of a length adjustable construction, is provided and suitably secured within the interior of the magazine box. At least a portion of the belted ammunition loops rest on a top side surface of the installed support bracket structure. The top support bracket structure side surface is (1) laterally sloped, in a direction transverse to the lengths of the belt loops, to inhibit a tilting-created intermeshing of vertically adjacent projectile ends of the ammunition rounds which could cause a feed jam, and/or (2) longitudinally sloped, in a direction parallel to the lengths of the belt loops, to desirably lessen the ammunition outfeed force that must be exerted by the machine gun.