The Colt ACR (also known as the M16A2E2) was Colt's entry in the Advanced Combat Rifle program, which concluded with the result that none of the entrants in Phase III achieved enough of an improvement over the M16A2 to be worth the cost.
The weapon was not a total loss, however; the optic 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 ACR is based on the M16A2 platform, with modifications made to the design. The first is a muzzle brake/compensator system meant to reduce muzzle rise, along with an oil-spring buffer designed to lower perceived recoil of moving parts during automatic fire. It also has a six-position telescopic buttstock.
The first version of the weapon had a handguard similar to that of the M16A2 with an elevated track sight rib. The later versions of the ACR also featured a redesigned version of the handguard that features a distinctive sight rib boasting lightening holes and a white stripe for improved fast target acquisition.
The ACR could use either standard iron sights, or an optic scope. After the ACR program, a version of the optic scope would come to be adopted as the M145 Machine Gun Optic.
The key aspect of the weapon was the use of "duplex rounds", a single cartridge with two bullets in it. The Olin Corporation produced three different variations of this round for testing. The first consisted 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 steel-cored 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 at a time. However, they significantly reduced accuracy, requiring the user to also carry standard ammunition for longer-range engagements.