The Colt LE6921 M4 Upper Receiver Conversion Kit is a complete centerfire upper assembly chambered in 5.56mm NATO, replicating standard-issue military and law enforcement short-barreled configuration profiles. This factory-assembled kit provides a direct, drop-in option to convert any standard AR-15 lower receiver group into a battle-grade Colt M4 tactical weapon system. It features a high-grade 14.5-inch chrome-lined carbon steel barrel with a military step-cut contour, operating on a standard carbine-length direct gas impingement system. Colt LE6921 5.56mm 14.5" Bbl Matte Black Upper Receiver Conversion Kit LE6921CK