Out of stock
SKU: HK51000384
UPC: 642230270213
MPN: 51000384
The Heckler & Koch MR556A4 upper receiver kit delivers the same piston-driven operating system trusted by elite military and law enforcement units worldwide. This complete 16.5-inch assembly allows shooters to retrofit any Mil-Spec 5.56mm AR-style rifle to HK's proprietary piston and pusher rod design, replacing the traditional direct-impingement gas tube. Chambered in 5.56x45mm NATO and shipping with a 30-round magazine, this upper provides a direct path to HK416-level reliability without replacing your existing lower receiver.
HK engineered this upper around a 16.5-inch cold-hammer forged barrel with a 1:7 twist rate and six-groove rifling profile. The piston system keeps hot carbon fouling and waste gases out of the receiver area entirely—a fundamental departure from direct-impingement designs where combustion byproducts cycle directly through the action. The kit includes the upper receiver with forward assist, an extended convertible charging handle, and HK's Free Floating Rail System handguard. Overall length measures 25.94 inches with a width of 3.07 inches and height of 2.68 inches. The optics-ready configuration accepts your choice of sighting systems. For builders exploring other premium upper receiver options, we stock alternatives from several manufacturers.
This upper suits shooters who want enhanced reliability in adverse conditions without surrendering their existing AR lower investment. The closed-system design reduces cleaning frequency and virtually eliminates the carbon-related malfunctions common in standard gas systems. At 4.20 pounds, it adds meaningful capability without excessive weight. Whether you're building a duty rifle, a home defense platform, or a hard-use range gun, the MR556A4 upper delivers proven German engineering. Browse our full selection of 5.56 upper assemblies to compare configurations.
The MR556A4 upper fits any Mil-Spec AR-15/M16/M4-style lower receiver chambered for 5.56mm NATO. The takedown pins and receiver dimensions follow standard AR specifications, so it mounts directly to most commercial and mil-spec lowers without modification.
Direct-impingement rifles route hot combustion gases through a tube directly into the receiver to cycle the bolt. The HK piston system uses a separate piston and pusher rod, keeping carbon fouling and waste gases out of the receiver area. This results in a cleaner action and improved reliability.
Yes. This is a complete upper receiver kit that includes the barrel assembly, upper receiver with forward assist, extended convertible charging handle, HK Free Floating Rail System handguard, and all components needed to function. It ships with a 30-round magazine.
The cold-hammer forged barrel features a 1:7 right-hand twist with six-groove rifling. This twist rate stabilizes heavier 5.56mm projectiles effectively, including 62-grain and 77-grain loads commonly used for precision and barrier penetration.
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