A unique aspect of the potent .300 Blackout Caliber is that you can use existing .223/5.56 Bolts, Carriers and Magazines in your rifle as long as you have this Windham Weaponry .300 Blackout Flattop Upper Receiver Assembly. The complete Assembly includes a 16” Medium Profile Barrel machined from 4150M Chrome Moly Vanadium 11595E Steel. It is Chrome Lined in Bore and Chamber, rifled 1 turn in 7 inches – right hand twist and includes a 1913 Railed Gas Block just ahead of the M4 Double Heat Shield Handguards, and an A2 Birdcage type flash hider at the muzzle. The flat top type Upper Receiver is CNC machined from 7075T6 aircraft grade aluminum, hard coat black anodized to mil spec. This assembly is ready to attach to your lower receiver assembly and use with your Bolt, Carrier, Charging Handle and Magazines. The .300 Blackout Caliber ammunition will function in .223 / 5.56mm magazines, but for safety’s sake do not mix up the two ammunitions – which is why we Laser Engrave the Upper as .300 Blackout Caliber. Chambering incorrect ammunition in your upper could cause catastrophic damage or injury. Always use factory ammunition that corresponds to the caliber stampings on the barrel.