An ordnance mounting system for a helicopter operatively supports a weaponry support beam above an upwardly projecting floor obstruction in a cabin area of the helicopter, between opposite cabin door openings, with the support beam longitudinally extending transversely through the cabin area, and outer weaponry mounting end portions of the support beam projecting outwardly through the door openings beyond opposite fuselage side walls of the helicopter. The mounting system may be secured to door opening sill structures, or the cabin ceiling or side walls, and permits vertical bending flexure of the installed support beam due to flight and weaponry weight loads on the outer ends of the support beam.