In the late 1980s, the United States Army undertook a program to identify a potential successor to the long-running Colt M16 series of automatic rifles - the standard issue rifle of American armed forces and many allies. The program was recognized as the "Advanced Combat Rifle" (ACR) program with the primary goal being dramatic improvement in "first hit" probability. By this point in history, the M16 had been in frontline service nearly thirty years including a rather troublesome appearance during the Vietnam War (1955-1975) and later improved through the M16A2 model. The program spanned across three major phases (known simply as Phase I, Phase II and Phase III) and included submissions from major firearms manufacturers such as AAI Corporation, Heckler & Koch, Steyr of Austria and Colt. The Steyr submission was the "Steyr ACR" and this weapon was devised around the firing of a 5.56x45mm SCF (Synthetic Case Flechette) down a smoothbore barrel.
For its ACR, Steyr elected to use a similar bullpup configuration resembling their popular Steyr AUG assault rifle series. The bullpup arrangement merely identified the ACR as a weapon with its action and magazine feed located aft of the trigger group. Such an arrangement then allowed a full-length barrel to be utilized though in a more compact assault weapon body - the action nestled within the enlarged stock which was tucked away into the shoulder when firing. As with the AUG, the ACR was given a largely plastic body with metal internals to minimize both manufacture cost and overall carry weight. The weapon body included a two-piece plastic covering, one set over the main portion of the frame and the other housing the stock. The grip and trigger group were centralized along the bottom of the frame. The forend was wholly shrouded over and doubled as the foregrip and heatshield. The barrel muzzle was slotted in the M16 style and protruded only a short distance from the plastic body. Straight, proprietary plastic transparent magazines were inserted into the base of the stock, a magazine release button just aft of the feed well. Sling loops were located at the top of the shoulder stock and along the left side of the forend for shoulder strap usage. Over the top of the rifle was a two-gap rib assembly which acted primarily as a basic sighting device and could serve as an ad hoc carrying handle. To this assembly could be fitted optics as needed. A pull-type, spring-loaded charging handle was set at the rear of the sighting assembly as in the M16 and simply pulled back to cock the weapon. Spent cartridges were ejected through a port found under the stock between the magazine well and pistol grip. Overall, the ACR proved a clean and somewhat futuristic assault weapon design worthy of 21st Century warfare. As with the M16, the Steyr ACR utilized a gas-operated system in which a sleeve, wrapped around the barrel assembly, was driven rearwards during the action and this was mated to a vertically-sliding chamber managing the cartridge function.
The ACR measured a running length of 30.7 inches with an unloaded weight of 7lbs (the former made possible by the bullpup configuration and the latter made possible by heavy use of plastics). The barrel was not rifled and instead left as a smoothbore type and measured 21 inches in length. The detachable box magazine contained 24 ready-to-fire flechette rounds in a typical stacked arrangement. The base firing action of the ACR allowed for semi-automatic, three-round burst-fire.
One of the more unique aspects of the ACR design was its ammunition - essentially a plastic transparent case housing a steel, fin-stabilized flechette (dart) within a sabot-type housing. The plastic case held the flechette, sabot and propellant and was ultimately jettisoned downwards from the weapon during the firing action as normal. Each flechette measured 1.6 inches in length. Once the propellant was actuated by the striker pin, the sabot and dart proceeded down the barrel in the usual way. Once exiting the barrel at the muzzle, the sabot fell away. The flechette then continued on in the desired direction (of course the falling sabot tended to present a danger to nearby friendlies all its own, particularly with the exiting speeds involved). Despite its vast departure from traditional powder-based, bullet-tipped cartridges, this sabot-flechette approach actually acquitted itself quite well during the US Army tests, their high-velocity exit allowing for little change in overall trajectory and, therefore, increased accuracy. Overall muzzle velocity was an amazing 4,700 to 4,900 feet per second compared to the M16's 3,100 feet per second force.
In the end, none of the submitted assault rifle designs satisfied US Army authorities enough to warrant official adoption. Increases to first hit capability proved marginal, troublesome or too complex when compared to the standardized M16A2 models then in widespread service. It is, however, conceivable that data collected through the ACR's evaluation may someday influence a Steyr flechette-oriented assault weapon design still to come.