While of relatively common practice today, the idea of a single common assault weapon receiver modified to fulfill various battlefield roles was something of a novel concept in the 1960s where dedicated weapons were being developed to fulfill dedicated service in various militaries around the world. The Stoner 63 (or "M63") was, therefore, something of a novel concept for its time and pioneered by Eugene Stoner - maker of the AR-15 assault rifle which, in turn, became the U.S. Army's famous M16 series. The Stoner 63 utilized a common receiver core to which various feeds, barrels, shoulder stocks, and other components were attached to configure the weapon's in-the-field service. Internally, the system still relied on the proven gas-operated, rotating bolt action - a similar arrangement seen in the AR-15 - though outwardly, the weapon could take on various profiles to suit the mission need. Design work began in 1962 and support subsequently came from Cadillac Gage as manufacturer. Design attribution was also given to L. James Sullivan and Robert Fremont.
The original weapon model was the Standard Assault Rifle (SAR) fitting a 30-round detachable box magazine and operated as a basic automatic service rifle. Then came the Carbine (CAR) which was of a more compact, shortened length (both barrel and forend were shortened) with a folding shoulder stock to boot. The Automatic Rifle (AR) was fed by way of a 30-round detachable box magazine inserted at the top of the receiver with relocated sights to compensate for the feeding. The Light Machine Gun (LMG) model supported linked-belt feeding with a quick-change barrel facility as well as folding bipod - its battlefield role similar to the modern-day "Squad Automatic Weapon" (SAW). The Medium Machine Gun (MMG) variant allowed for the weapon's mounting onto a U.S. military tripod for the suppression role. The Fixed Machine Gun (FMG) mark lost its iron sights and pistol grip facility so actuation was done by solenoid for firing from armored vehicles (as a coaxial weapon). The "Commando" (COM) mark was fed by a 100-round drum magazine and operated in similar fashion as the LMG. The "Survival Rifle" (SR) was intended for aircrews as a survival weapon should their aircraft be downed in enemy territory. It was purposely compact with a short barrel and forend and lacked a handguard. Only a sole prototype of this model completed.
To produce one weapon form from the other appropriately required modifications to be completed by the operator. All forms were chambered for the ubiquitous 5.56x45mm NATO standard cartridge which benefitted logistics and cost. The gas system sat in a cylinder under the barrel in the usual way. Effective engagement ranges (depending on configuration) reached out to 1,000 meters with a maximum firing range listed at 2,655 meters.
First-batch production numbered 2,000 units and the weapon was formally trialled by the United States military, first through DARPA (Defense Advanced Research Projects Agency) beginning in March of 1963 and then, later that year, by the United States Marine Corps (USMC) which returned with some modifications, producing the "Stoner 63A" designation of 1966. An additional 2,000 or so guns were produced to this standard. The U.S. Army also showed limited interest and scheduled trials all their own but were not sufficiently impressed with the concept of an "all-in-one" gun and found fault with the action and overall concept. Additionally, the usually lighter rifle and carbine forms were heavier than competing types for the Stoner 63 receiver core was also the basis of the usually heavy-duty light- and medium-machine gun forms. There proved no real workaround for this.
Perhaps its best known use was in the hands of the Navy SEALs during the Vietnam War from 1967 onwards. The SEALs took on several experimental and developmental weapons during this time and pressed them hard in their combat roles - the perfect testing stage and client for a new gun. Its use here, however, did not lead to widespread acceptance of the system nor the concept. U.S. Army Special Forces took on the light machine gun form as the "XM207" from 1970 onwards but this only last about one year and did not land an endorsement. Due to the varied models and test forces, various designations were applied to the weapon throughout career - XM22, XM22E1, XM22E2, XM23, XM23E1, XM207, XM207E1, XM207E2, and Mark 23 Mod 0.
Amazingly, the Stoner 63 series soldiered on into the 1980s but it was eventually (and officially) written off as the U.S. military continued its love affair with dedicated weapons for dedicated roles. NWM of the Netherlands purchased the rights to the gun for manufacture and sales outside of the United States but the product found no takers on the world stage - its non-endorsement by the U.S. military certainly hurt its cause.
The universal receiver concept has never formally gone away - several large concerns and private companies have sought new approaches to the concept with few seeing widespread acceptance. The Austrian Steyr AUG probably remains the most famous - its common receiver making up an assault rifle, light machine gun, and commando weapon form.