The AAI ACR Assault Rifle (also called the AAI Low Impulse Steyr ACR, as part of the design includes an internal anti-recoil device) was based on AAI’s previous work during the SPIW program of the 1970s. The AAI ACR design presented for evaluation outwardly looked almost completely conventional, but was quite unconventional in many ways. Like all the other Steyr ACR candidates of the time, the AAI ACR was rejected by the US Army and became a museum piece.
The 18.5-inch barrel was tipped by a compact pepperpot-type muzzle brake; the bore used a very lazy twist rate (1:85), since the ammunition was essentially self-stabilizing. Most of the was of steel or light alloy, but the stock, fore-end, and pistol grip were of polymer/plastic construction using materials that were advanced for the time. (Early versions of the AAI ACR Assault Rifle did not have a pistol grip, but instead a pistol grip wrist.) The fire selector used a 3-round burst mechanism that fired at a cyclic rate of 1800 rpm so fast that the third round would be well downrange before the shooter would feel the recoil from the first round.
This ACOG, though roughly twice as large as modern ACOGs, set the stage for future developments. The ACOG had 4x magnification and limited night vision, and even worked well at night. Standard adjustable iron sights were also developed, with the rear sight assembly being removable and fitting onto the receiver’s sight base, and a low sighting rib was also found above the barrel for quick shooting.
The ammunition that AAI used was based on flechette rounds developed well before the SPIW program. The muzzle velocity of the flechettes was very high (over 1400 meters per second), and the flechette had excellent penetration. The flechette (like most flechettes) twisted into a fishhook-shape upon striking a person, causing wounds out of proportion to the size of the flechette so much so that it was briefly thought that AAI’s round might be a violation of the Geneva Accords.
However, the AAI flechette was not without its problems; the long, finned, needle-like shape (about 1.6x41mm) together with its very light weight (about 0.56 grams) made it extremely susceptible to wind. The round, nestled in its casing and liquid-crystal boot, was almost identical in size to the 5.56mm NATO round, and the magazines themselves were based on M-16-type magazines. The AAI ACR could not fire 5.56mm NATO rounds, though – doing so would cause a chamber explosion, usually accompanied with the bolt assembly blowing backwards out of the weapon at high speed, possibly injuring or even killing its shooter.
The M-16-based magazines were quickly modified before such an accident could happen so that one could not load 5.56mm NATO rounds into AAI ACR magazines and standard M-16-type magazines would not fit into the AAI ACR. (A 62-round drum was also developed for the AAI ACR, as the company planned to develop a whole family of small arms based on its ACR if the military decided to adopt it – including a SAW.) The problems with the ammunition were one of the main strikes against the AAI ACR; in addition, the cost per round was very high.