In the late-1970s, the United States Army began looking at the prospect of succeeding its M16A2 combat rifles and this was the impetus for various small arms weapons programs of the 1980s and 1990s that followed. In the 1980s, this was the Advanced Combat Rifle (ACR) program which sought to increase the first-hit probability of the standard infantryman. Three phases were fleshed out and six contractors were part of the initial work. By Phase III, only AAI, Colt, Heckler & Koch and Steyr were left.
Colt elected for an advanced form of its existing M16 rifle family and, as such, the weapon held a look and feel similar to that of the earlier rifle. The greatest change was in the oversized heat shield added over the barrel component and a revised, longer-running carrying handle over the receiver. The buttstock was collapsible and a slotted flash hider fitted over the barrel. An ambidextrous selector control scheme allowed for single- or full-automatic fire. Beyond the iron sights fitted there was provision for an optics set to be installed. Chambering was in 5.56x45mm NATO or "Duplex" while the operation remained gas-operated at its core.
The Duplex round was a single cartridge holding two bullets and developed by the Olin Corporation (Winchester) for the competition. The company eventually pushed three different cartridge types with the third offering being selected for finalizing. In the Duplex design, multiple bullets were sent at the target in quick succession, theoretically improving first-hit probability. The first arrived where aimed with the second arriving near the first's target zone. Many of the other ACR submissions followed the multiple bullet approach as this was deemed the best possible method to fulfill the Army's requirement.
While the muzzle compensator worked well to reduce recoil and maintain control of the firing weapon, the dual-bullet design reduced accuracy at long ranges - requiring the operator to also carry basic 5.56x45mm NATO ammunition for any shots longer than medium range (about 325 meters).
In the end, none of the submissions were selected for further development as none could increase the M16A2's first-hit probability by 100%. After several hundreds of millions of dollars, the ACR program was killed. Attention then turned to the Objective Individual Combat Weapon in the 1990s but this too failed to find a successor to the storied M16 family of rifles - which remains in use today (2016).