The MR556 Upper Receiver Kit has been upgraded to incorporate all the latest A4 enhancements, including a redesigned barrel profile, adjustable gas block, updated handguard, and more.
This kit features a complete upper receiver and barrel assembly, enabling Mil-Spec 5.56mm AR-style rifles to be converted to HK’s advanced short-stroke gas piston system. Unlike traditional direct-impingement systems, this closed system uses a piston and pusher rod instead of a gas tube, significantly reducing malfunctions by keeping hot gases and carbon fouling out of the receiver. This is the same proven system used in the HK416, trusted by elite military and law enforcement units worldwide.
The MR556 A4 is assembled in the USA using a combination of American and German components. It shares many parts and assemblies originally developed for the HK416 and delivers reliable performance and accuracy with a wide range of quality 5.56mm or .223 ammunition.
MR556 A4 Upper Kit Includes:
- Complete MR556 upper receiver/barrel assembly with forward assist, extended charging handle, and free-floating slimline handguard
- Complete MR556 bolt group assembly
- HK proprietary buffer and action spring
Note: The HK buffer and action spring are shorter than standard and are not compatible with fixed-stock AR/M16 rifles. For reliable operation, Mil-Spec hammers must be installed on non-HK lower receivers.
