I photographed this Stoner 63 Gun System display at the Virginia War Museum; Newport News VA
Eugene Stoner created the 63 system so that a variety of weapons could be built on a common base. His system had 15 possible variations that all fired the 5.56mm M-16 round from a magazine or belt — configurations such as Light Machine Gun with Belt, Assault Rifle, Carbine, Squad Automatic Rifle and Co-axial Fixed Machine Gun.
As a matter of poor timing, the problems with the M-16 were being resolved at around the time he was making his pitch, and the military was not ready to take on a new system that might experience issues.