The Stoner 63 Weapon System is the brain child of Eugene Stoner, the chief designer of the AR-10, AR-15/M-16 and many other unique weapon designs. The project goal was to develop a family of small arms built around a common receiver and a number of assemblies. The precursor of the Stoner 63 weapon series was the M69W in 7.62X51mm. The M69W designed in 1962 was the first design to employ a common receiver that could be inverted for use as a light machine gun (LMG) or a rifle. The second generation was given the designation Stoner 62 and was again in 7.62X51mm. Only two were generated before Cadillac Gage decided to focus on the 5.56X45mm round. The Stoner 63 was finally ready in 1963, hence the name. The complete Stoner 63 weapons system was demonstrated to the Marines in August of 1963. L Company 3rd 1st Marines (Capt. Joseph Gibbs Commanding) tested the Stoner in Vietnam for approximately six months. The Marines were impressed with the light machine gun and rifle configurations. The Marines use Army Material Command for logistical support and therefore were selected to perform trials on the Stoner 63. Unfortunately the Army Material Command was prejudiced against previous Stoner designs and they were already determined to adopt the Rodman bull pup SAW using the 6mm XM732 cartridge. The tests were conducted at the Rodman Lab test facilities and the project manager for the Stoner 63 was also involved with the Rodman project. The Stoner 63 failed the test due to having unacceptably high number of stoppages and failures. This was attributed to Army Material Command imposing unrealistic ammunition requirements that exhibited large variations in gas port pressure. After the failed test, Cadillac Gage was requested to submit the Stoner 63 in the 6mm XM732 round but they declined.
The Stoner 63 went to Vietnam while barely past the prototype stage. Even at this early developmental stage the SEALs found it to be a extremely useful weapon in combat. Before the Army Material Command could complete their prejudiced test, the U.S. Marines, Army, and Air Force submitted recommend changes to the Stoner 63 system. A total of 2,400 Stoner 63s had been manufactured up until this point. The modifications were implemented in August of 1966 and the weapon system was redesignated as the Stoner 63A. The Stoner 63A now limited issue to the SEALs was designated as the XM23 for the carbine, XM22E1 for the rifle and XM207 (later the XM207E1 then Mk23 Mod. 0) for the light machine gun. NWM (Nederlandsche Wapen-En Munitiefabriek) of the Netherlands acquired the rights to sell the weapon outside North America in 1967. NWM introduced some modifications in 1969 and redesignated the weapon as the Stoner 63A1. NWM never manufactured any Stoners. All NWM Stoners were made by Cadillac Gage and exported to Europe. The NWM guns were later imported back to the United States as Pre-86 dealer samples. Stoner production ceased by 1971 with 2,400 63s, 850 63As, and 100 Mk23 Mod 0 produced. Most of these guns were torched by the government after the adoption of the FN M249 in 1983. Reed Knight bought the tooling from Cadillac Gage and produced 100 receivers before the 1986 machine gun ban. The low production numbers coupled with the lack of US adoption has made the Stoner Machine guns rare, but much sought after, NFA weapon. There are approximately 25 transferable Stoner 63s left. Registered Stoner 63As consist of approximately 35 pre86 dealer samples and 5 transferables, not including the 100 transferable ones manufactured by Reed Knight.
(source) Mongo's Machine Gun page