Ideal for crafting your personalized AR-15 upper assembly, this product features a meticulously installed M4 barrel extension, ensuring optimal performance and reliability. Each barrel sub-assembly is expertly head spaced to guarantee precision and safety, providing you with a solid foundation for your custom build. Multi-role profile 4140 chromium molybdenum steel Internal and external surfaces feature a salt bath nitride finish Twist rate - 1:7 Chamber for .300 AAC Blackout Threaded 5/8-24 3/4 inches gas journal