HK MR556 Upper Receiver Kit for AR15/M16/M4, Piston System 5.56mm, 51000385

Product Description The HK MR556 Upper Receiver Kit is a complete upper receiver/barrel assembly designed to retrofit Mil-Spec 5.56mm AR-style rifles (AR15/M16/M4 pattern) to HKs piston and pusher rod operating system. By replacing the traditional direct-impingement gas tube with a closed piston system, this design helps keep hot carbon fouling and waste gases out of the receiver area for cleaner operation and improved reliability. This HK piston-driven concept is the same key feature that contributed to the success of the HK416 platform used by military and law enforcement customers. The upper ships in an HK soft case for storage and transport. Product Highlights Complete HK MR556 upper receiver/barrel assembly to retrofit Mil-Spec 5.56mm AR-style rifles Closed piston and pusher rod system reduces carbon and gas blowback into the receiver vs. direct impingement Designed for AR15/M16/M4 pattern compatibility (Mil-Spec 5.56mm) Kit shown with bolt carrier group, buffer, and action spring components (see product image) Ships in an HK soft case for protected storage and transport Specs Brand: HK Product Type: Upper Receiver Kit (Upper receiver/barrel assembly) Platform Fit: AR15 / M16 / M4 (Mil-Spec AR-style rifles) Caliber: 5.56mm Operating System: Piston and pusher rod (closed system; replaces standard gas tube) Included/Shown Components: Upper receiver/barrel assembly; kit shown with bolt carrier group, buffer, and action spring (see image) Case: HK soft case (included) Manufacturer Part Number: 51000385 UPC: 642230270220 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 products manufacturer.