Abstract: An elongated support plank structure longitudinally extends transversely through the cabin area of a helicopter and has outer end portions projecting outwardly beyond opposite sides of the helicopter. A pair of 30 mm machine guns are mounted on the plank end portions and are supplied with belted ammunition from magazine boxes mounted on the plank within the cabin area via flexible feed chutes interconnected between the magazine boxes and interiorly rollered feed adapters interconnected between the outer ends of the feed chutes and the inlets of the machine gun feeder mechanisms. The 30 mm machine guns are secured to the outer plank end portions by specially designed mounting structures each having a cradle portion with axis-adjustable roller assemblies that engage the gun and essentially preclude lateral movement thereof relative to the cradle, but permit roller-guided longitudinal gun recoil movement, during firing of the gun.