Steyr ACR
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The Steyr ACR was a prototype flechette-firing assault rifle built for the US Army's Advanced Combat Rifle program of 1989/90. Wikipedia
Action: Gas-operated, vertically sliding chamber
Barrel length: 21.3 in (540 mm)
Caliber: 5.56mm (Sabot Diameter)
Cartridge: 5.56×45mm annular-primed SCF (synthetic case flechette)
Mass: 7.12 lb (3.2 kg) (unloaded)
Muzzle velocity: 4,757 ft/s (1,450 m/s)
Rate of fire: Semi automatic 3-round burst at 1200rpm cyclic rounds per minute