It has taken my about 17 years but I have finally found out what this freaking weapon is called:
It's the prototype Colt entered in the Army's old Advanced Combat Rifle (ACR) program in the late 80s.
The first time I ever encountered this weapons silhouette, was during the waning days of the G.I. Joe Hall of Fame toy line, because the Combat Camo Duke action figure and Green Beret weapons set came respectively with an olive green and silver. I think that was like 1993, so I must have been 5 or 6 years old at the time. As a child, it was actually a geeks curiosity about my toy's weapons, that drove me to study how weapons work, etcetera. This particular rifles model number always eluded me, it was an enigma that I never saw anywhere else. A stock reminiscent of an M4 tube, an ELCAN/C79 style scope, M16'esque magazine, but the rest became alien. Almost like an alternate evolution of the Colt M16A1 or an early version of the M16A2. Now I know why! It was made by Colt for the ACR program, and obviously based on the M16A2's it would have been replacing, if the program didn't flunk lol. Sadly it's successors also flunked.
Ironically not even knowing about the ammunition being tested with the Colt ACR, as a child I would often use it in toy battles as a substitute for an urban sniper rifle, in a role in between what you might see the old KAC SR-25s and modern Army SDM-R/USMC SAM-R rifles used for—but firing a special "Hyper (velocity) penetrator" sabot round. Still I would rather of had an MSG-90/PSG-1, even as a kid, I had a taste for Heckler & Koch.
Sometimes I wonder if I am part psychic, hehe.