Product Description The CMMG 5.7x28mm Barrel and Bolt Carrier Group (BCG) Kit is built for assembling an AR-platform upper in 5.7x28mm using CMMG&';s patented Radial Delayed Blowback operating system. Compared to typical direct blowback setups, Radial Delayed Blowback helps reduce bolt carrier group weight and delivers less felt recoil while maintaining reliable function. This kit is designed to assemble into a mil-spec AR-15/Mk4 upper and is compatible with a CMMG Mk57 lower, or a standard Mk4/AR-15 lower when paired with CMMG 5.7 AR conversion magazines. Product Highlights 8-inch 5.7x28mm barrel and matching 5.7x28mm bolt carrier group kit for AR-platform builds CMMG Radial Delayed Blowback system for reduced felt recoil versus straight blowback designs Designed for mil-spec AR-15/Mk4 upper receiver assembly Works with CMMG Mk57 lower or standard AR-15 lower (with CMMG 5.7 conversion magazines) Black finish to match common AR build aesthetics Specs Brand: CMMG Product Type: Barrel & Bolt Carrier Group (BCG) Kit Caliber: 5.7x28mm Barrel Length: 8 in Color/Finish: Black Compatible Platforms: Mil-spec AR-15 / Mk4 upper; CMMG Mk57 lower or AR-15 lower with 5.7 conversion magazines Operating System: Radial Delayed Blowback UPC: 810097502406 Manufacturer Part Number: 57D88C3 Disclaimer: We strive to provide accurate and up-to-date product descriptions, specifications, and images; however, errors or omissions may occasionally occur. We are not responsible for any inaccuracies or discrepancies in the content provided. If you notice anything incorrect or unclear, please contact us so we can correct it. When in doubt, we recommend verifying details directly with the product&';s manufacturer.