The AR-STONER 12.7x42mm Heavy Contour was created for those who want a big bore AR-15. The heavy contour barrel helps keep the felt recoil and muzzle climb minimal. The M4 feed ramps helps the round chamber consistently giving you the reliability you expect. Finish building the gun of your dreams at a price that won't break the bank with the AR-STONER(TM) AR-15 A3 Upper Receiver Assembly.FeaturesHeavy contourM4 feed rampChambered in 12.7x42mmNote: 12.7x42mm is the metric designation for the 50 Beowulf. 50 Beowulf is a registered trademark of Alexander Arms.