The Heckler & Koch MR556 A4 Upper Receiver is a complete upper assembly designed to retrofit Mil-Spec .223 Rem/5.56 NATO AR-style rifles, featuring HK’s legendary short-stroke gas piston system that virtually eliminates carbon fouling and dramatically increases operational reliability. The kit includes a 16.5-inch cold hammer forged barrel, complete upper receiver with forward assist, and an extended convertible charging handle. The receiver features a 1:7″ twist rate and suppressor-ready configuration with a monolithic M-LOK handguard and full-length Picatinny top rail.
Built to the same standards as the HK416 used by elite military and law enforcement units, this upper is engineered for shooters who demand top-tier performance. The piston and pusher rod system operates in place of the gas tube normally found in AR15/M16/M4-style rifles, and this closed system virtually eliminates the malfunctions common in direct-impingement gas systems. The kit includes a complete MR556 bolt carrier group, proprietary buffer system, adjustable gas block, and soft transport case.
The MR556 A4 is assembled in the USA using US and German-made components and uses many of the same parts and assemblies originally developed for the HK416. It functions reliably and shoots accurately with a wide variety of quality 5.56mm or .223 ammunition. Whether you’re building a duty carbine, suppressed rifle setup, or upgrading your existing platform, this complete upper delivers the precision and reliability HK is known for.
