The Colt ACR was Colt's entry in the U.S. DoD Advanced Combat Rifle program, which concluded with the result that none of the entrants achieved enough of an improvement over the M16 to be worth the cost.
The Colt ACR was not a total loss, however; the scope went on to be offered by Elcan, a version of which was eventually adopted as the M145 Machine Gun Optic used on a number of firearms, especially those from Diemaco.
The first design for the Colt ACR looks very similar to the M16A2, but was later changed to a design that bears a resemblance to the AR-15 and inspired the design for the M4 carbine.
The key design change was the use of "duplex rounds", a single cartridge with two bullets in it. Olin Corporation produced three different rounds for testing, the first consisting of two tungsten projectiles in a long-necked case, the second used a standard-length case with two 27 grain (1.75 g) 0.158-inch (4 mm) tungsten projectiles, and the final entry was another standard-length case with two 0.224-inch (5.69 mm) projectiles, one 35 grains, (2.27 g) the other 33 (2.14 g). The latter was eventually selected for submission to the ACR trials. The basic idea of the duplex load is to increase the amount of projectiles fired. However, they significantly reduced accuracy, requiring the user to also carry traditional ammunition for long-range shots.