HK MR556 A4 Upper Kit | 5.56/.223 | 16.5 in | 51000384

The MR556 A4 upper kit from Heckler & Koch adapts Mil-Spec AR-15 platforms to HK’s piston-and-pusher-rod operating system, delivering a closed gas system designed to reduce hot carbon and gases entering the receiver area. This complete upper receiver and barrel assembly ships with a slim free-floating M-LOK handguard, extended charging handle, and a complete bolt group. The kit is assembled in the United States using U.S. and German-made components and is built for reliability across a wide range of 5.56mm/.223 ammunition.

Product Specifications

• The manufacturer is Heckler & Koch (HK).
• The model is the MR556 A4 Upper Receiver.
• The manufacturer’s part number is 51000384.
• The product UPC is 642230270213.
• The listed caliber is .223 Rem / 5.56 NATO.
• The barrel length is 16.5 inches.
• The barrel material is steel.
• The barrel finish is stainless.
• The exterior finish/color is black anodized.
• The upper is designed to fit AR-15 style lowers.
• The listed weight is 11.3 pounds.
• The handguard is a free-floating slimline M-LOK design.
• The kit includes a complete bolt group and HK proprietary buffer and action spring.
• Notable features include a forward assist and an extended charging handle.
• The barrel is 1/2 x 28 threaded (suppressor-ready).

The MR556 A4 brings HK’s piston operating system and a robust parts package to shooters who want a hardened upper for demanding use. With a free-floating M-LOK handguard and a complete bolt group included, this kit is suitable for training and duty-style applications where controlled, reliable operation and ease of maintenance are priorities.

Owners commonly praise the MR556 A4 for its reliable operation. Many also note its consistent accuracy across common 5.56/.223 loads.