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.
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