HK 51000384 MR556A4 5.56 NATO 16.5" Upper Receiver Kit

The HK MR556A4 5.56 NATO 16.5" Upper Receiver Kit 51000384 is a premium piston-driven upper designed to convert standard AR-15 lowers into HK’s reliable short-stroke gas system, similar to the HK416. HK 51000384 features an 16.5" cold hammer-forged barrel (1:7 twist), an adjustable gas regulator for suppressor use, and a slim free-floating M-LOK handguard with a full-length Picatinny rail.

The kit includes the complete upper, HK bolt carrier group, buffer, buffer spring, extended ambidextrous charging handle, and a soft case. It’s compatible with most mil-spec AR-15 lowers but requires a mil-spec hammer and may not work with fixed-stock setups. This setup is ideal for users seeking enhanced durability, reliability, and suppressor readiness.

The HK MR556 Upper Receiver Kit has been upgraded with the latest A4 features, including a redesigned barrel profile, adjustable gas block, enhanced handguard, and more.

This HK MR556 Upper Receiver Kit allows Mil-Spec 5.56mm AR-style rifles to be converted to HK’s proven short-stroke gas piston system. Unlike traditional direct-impingement designs, this system uses a piston and pusher rod to cycle the action, keeping hot gases and carbon fouling out of the receiver. The result is cleaner, cooler operation and increased reliability—critical advantages shared with the renowned HK416, trusted by elite military and law enforcement units worldwide.

Assembled in the USA using a combination of German and U.S.-made components, the MR556 A4 upper offers exceptional accuracy and dependable performance with a wide range of 5.56mm or .223 ammunition.

The HK MR556 A4 Upper kit includes

• 1. Complete MR556 upper receiver/barrel assembly, barrel, upper receiver with forward assist, extended charging handle and free-floating slimline handguard.


• 2. Complete MR556 bolt group assembly


• 3. HK proprietary buffer and action spring