HK MR556A4 11" Upper Receiver Kit w/Bolt, Buffer Spring, M-LOK, Supp Ready, Adj Gas Regulator, Pic Rail, & Ext Ambi Charging Handle 51000385

The HK MR556 A4 Upper receiver kit brings HK's proven piston operating system to AR-platform rifle owners who want to retrofit their existing Mil-Spec lower receiver with the same operating system technology that makes the HK416 the trusted choice of elite military and law enforcement units worldwide — all the new A4 features including an updated barrel profile, adjustable gas block, new handguard, and more in a complete kit that transforms a standard direct impingement AR into an HK piston-operated platform.

The MR556 upper kit allows Mil-Spec 5.56mm AR-style rifles to be retrofitted with HK's piston-and-pushrod system, a closed operating system that uses a piston and pushrod in place of the gas tube normally found in AR-15, M16, and M4-style direct-impingement configurations. This closed system virtually eliminates the malfunctions that direct-impingement gas systems are susceptible to, hot carbon fouling, and waste gases are not propelled back into the receiver area to operate the action, keeping the bolt carrier group significantly cleaner and cooler during extended shooting sessions and suppressed use.

This is the same key operating system feature that is critical to the success of the HK MR556A4 and the HK416 used by elite military and law enforcement customers who depend on reliable function in the most demanding operational environments where direct impingement reliability limitations become practically meaningful.

The HK MR556 Upper A4 kit is assembled in the USA using US- and German-made components, utilizing many of the same parts and assemblies originally developed for the HK416. The platform functions reliably and shoots accurately with a wide variety of quality 5.56mm and .223 ammunition across the full range of loads that AR platform shooters regularly run.

The MR556 A4 Upper kit includes a complete MR556 upper receiver and barrel assembly with forward assist, extended charging handle, and free-floating slimline handguard — plus a complete MR556 bolt group assembly, HK proprietary buffer, and action spring.