Add the stopping power of the .300 Blackout Caliber to your rifle with this Windham Weaponry Flattop Upper Receiver Assembly. The complete Assembly includes a 16” Medium Profile Barrel with Bolt, Carrier and Charging Handle. The Barrel is 4150M Chrome Moly Vanadium 11595E Steel, Chrome Lined in Bore and Chamber, rifled 1 turn in 7 inches - right hand twist. A 1913 Railed Gas Block is installed just ahead of the M4 Double Heat Shield Handguards, and the muzzle is fitted with an A2 Birdcage type flash hider. 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 take to the range. 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.