The Rock River Arms AR-15 Upper Receiver Assembly is designed to pin onto any mil-spec lower receiver. Just add an optic, or set of sights, and you are ready to send lead downrange! Features Flattop upper receiver Mil-spec compatible Anodized finish Cartridge 223 Wylde Color Anodized Black Material Aluminum Barrel Length 10.5 Inches Barrel Material 4150 Chrome Moly Steel Barrel Finish Black Twist 1:7 Inches Threads 1/2"-28 Sights No Chrome Lined No Gas Block Diameter 0.75 Inches Gas System Length Carbine M4 Feed Ramps Yes Upper Receiver Type A3 Flat-Top