Stoner 63

Stoner 63 is a modular weapons system developed by the late Eugene Stoner, who also designed the AR-15 series of weapons.
The goal of the project was to create a base that would have interchangable parts, so when needed, the weapon would be able to function as a belt-fed light machine gun or a magazine-fed assault rifle. Early evaluation versions were handed out to Navy SEALs during the Vietnam War, and the results came positive, with operators favoring the design of the weapon coupled with the effectiveness of the 5.56x45 mm round it used.
 
The Stoner 63 never saw mass production, as the project was scrapped and most of the existing firearms were destroyed.
The few remaining M63 models are prized collector's items. There is known to be a Stoner 63 within the KAC(Knights Armament Company, producers of  AR-15 variants and the M110 among other warfare items) armory.