In the mid/late 1960s, Colt was manufacturing AR-15 rifles and wanted to supply light machine guns to go with them - so they developed the CMG-2 ("Colt's Machine Gun"). The CMG-2 competed against the Stoner 63 in trials for the Navy SEALs (among others), and narrowly lost out. It was a very well designed and thought-out weapon, but not quite as good as the Stoner.
A few years later in the early/mid 70s, Colt brought the design back in response to a request for a belt-fed 7.62mm machine gun for special operations units. Something with the firepower of the M60 was desired, but in a lighter package. Colt took their CMG-2, scaled it up to 7.62mm NATO caliber, and redesignated it the CMG-3. A total of 5 guns were made, and they went into military testing.
Unfortunately for Colt, the design wasn't quite as simple to scale up as they had hoped. The CMG-3 was determined to have a service lifespan of about 35,000 rounds - a third of what was required. Around that point, the receivers would fracture at the front trunnion. By the time Colt had worked out a new design to strengthen the guns without adding too much weight, the contract opportunity had passed, and the improved version was never made.
Of the five guns originally made, I believe only two still exist. This one is serial number 1, and I was privileged to get permission of the consigner to test-fire it on camera. In my opinion, it is a fantastic gun. At only about 18 pounds it is remarkably light for a 7.62mm beltfed, and quite simple to shoot from the shoulder. As with the SMG-2, it's internals are a parade of clever elements.