We’ve posted about the ACR Program here on TFB before, but there’s a lot of information available on the subject through DTIC. The Advanced Combat Rifle program was begun in the late 1980s as a research and development effort which would eventually lead to the next U.S. service rifle, replacing the M16. The intention was to field the new weapon, whatever it would be, before the end of the 1990s. With the fall of the Soviet Union, the ACR program withered, and the successor to the M16 in U.S. Army service became the M4 Carbine, another derivative of the Colt AR-15.
With so many documents available on DTIC, it’s difficult to single out just one. Therefore, I urge readers to follow the link and start reading!
A basic overview of the Colt ACR program can be found in the excellent video briefing posted in the link above: