Picture of a Navy SEAL from during the Vietnam War, holding a Stoner 63A.
The Stoner 63 was the brainchild of Eugene Stoner, Bob Fremont, and Jim Sullivan, the old AR10, AR15, and AR18 gang. The vision was a modular receiver which one could bolt on different things to as to configure it for different purposes, rifles, carbines, machineguns, using magazines or belts, firing from a closed bolt or open bolt, etc. Initially developed as the Stoner 62, a 7.62x51mm NATO weapon, but by 1963 it was readily apparent that interest was shifting much more towards 5.56x45mm NATO and that patch was chosen instead.
The US Marines tested these and didn't find them satisfactory (partially due to unrealistic expectations and demand), but the Navy SEALs did grow the like them, the rifle somewhat, but *particularly* the belt-fed light machinegun configuration, they had never had a belt-fed squad automatic weapon in 5.56mm before, and they thought it was just THE coolest fucking thing, because the recoil is nothing and you can carry so much ammunition. There were 100rd belt boxes, but later 150rd belt drums and 200rd custom belt boxes would be used.
Even as the project was cancelled, the Navy SEALS kept these in inventory, having done a bit of development on it, and keeping it as the Mark 23 Mod. 0 (not to be confused with the later .45 caliber pistol by Heckler & Koch, given the same designation, for some reason). They kept these in use until the 1980s, when it was finally replaced by the FN Minimi as the M249.