HK MR556A4 5.56 NATO 16.5″ Upper Receiver Kit w/Bolt Buffer Spring M-LOK Supp Ready Adj Gas Regulator Pic Rail & Ext Ambi Charging Handle 51000384

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 includes1. 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 assembly3. HK proprietary buffer and action spring