Heckler & Koch MR556A4 Upper Kit 5.56 NATO 16.5″

Description

The Heckler & Koch MR556A4 upper receiver kit provides a complete upper assembly to retrofit Mil-Spec 5.56mm AR-style rifles to H&K’s short-stroke piston and pusher rod system. The closed piston system limits hot carbon and fouling entering the receiver compared with direct-impingement designs and is the same core operating principle used in the HK416 family. The MR556A4 incorporates updated A4 features including a new barrel profile, adjustable gas block, free-floating slimline M-LOK handguard, and an extended charging handle. The kit ships with the complete upper assembly, bolt group, and proprietary HK buffer and action spring.

Key Specifications

• The caliber is .223 REM/5.56 NATO.
• The barrel length is 16.5 inches.
• The barrel finish is STAINLESS.
• The barrel material is STEEL.
• The rate of twist is 1-in-7″.
• The model is listed as the MR556 A4 Upper Receiver.
• This upper is intended to fit AR-15 style lower receivers.
• The exterior finish is Anodized and the color is Black.
• The handguard style is M-LOK free-floating slimline.
• The kit includes bolt and buffer along with HK proprietary action spring.

Built to H&K design standards and assembled in the U.S. with U.S. and German-made components, the MR556A4 offers a durable, serviceable upgrade path for shooters seeking the reliability benefits of a closed short-stroke piston system. Its updated A4 features and included components make it a practical option for users who want a ready-to-install complete upper assembly with proven operating characteristics.