The AAI ACR (Advanced Combat Rifle) was a trials platform for the United States Army's ACR program of the 1980s. The program proceeded along three major phases which began with Phase I in February of 1986. AAI Corporation was contracted alongside Ares, Colt, Heckler & Koch, McDonnell Douglas and Steyr-Mannlicher to develop a successor to the storied, but limited, M16 Assault Rifle. The overall goal of the program was to find a weapon system of increased "first hit" probability - attempting to better the results of the M16A2 model specifically. Phase III of the program involved only AAI, Colt, H&K and Steyr.
The AAI submission became a refined form of its earlier flechette-firing rifle chambered for a sabotted 5.56x45mm cartridge (the flechette was 1.6x41.27mm). The flechette approach reduced recoil but muzzle blast and the noise footprint generated by the weapon when fired was such that the company was forced to add a flash hider/sound suppressor at the weapon's business end. A three-round burst feature was the standard, and only, mode of fire with the internal makeup of the gun believed to have a triple-chamber breech unit - allowing three rounds to be fired off in succession and thus increasing first-hit probability.
One of the major drawbacks of the AAI submission was the chamber able to accept regular 5.56x45mm NATO cartridges which, when coupled to the very different gas settings of the gas operated action, could lead to disastrous results for the gun and operator alike. As such, the magazines in question had to be proprietary to avoid loading the wrong type into the weapon and the magazines were further developed to only accept the intended cartridge. However, individual rounds could still be loaded into the chamber manually which still supported the issue.
On the whole, the weapon sported a traditional appearance, more conventional than its competitors, and held very clean lines at the receiver. The detachable box magazine was curved forward in the usual way and inserted ahead of the trigger group and action. Over the receiver was fitted standard iron sights but a quick-release optical set was also supported. The shoulder stock was solid and fixed at the rear of the weapon.
From 1989 to 1990 trials of the prototypes were held but none of the presented products fulfilled the 100% first hit probability requirement in full. The program was ended in April of 1990 and paved the way for the "Objective Individual Combat Weapon" program which followed. Similarly, this program failed to net an M16 successor at the cost of hundreds of millions of American tax-payer dollars.