H&K MR556 A4 Upper Receiver Kit 5.56 NATO

The Heckler & Koch MR556 A4 complete upper receiver kit allows Mil-Spec AR-15 style lowers to be retrofitted to HK's piston and pusher-rod operating system. This complete upper/barrel assembly includes a free-floating handguard, adjustable gas block, and HK-specific buffer and action spring to support reliable function. The closed piston system is designed to limit the introduction of fouling and hot gasses into the receiver area compared with direct-impingement systems.

Key Specifications

• Chambered for .223 Rem / 5.56 NATO.
• Model designation: MR556 A4 Upper Receiver.
• Designed to fit AR-15 lower receivers.
• Barrel material is steel.
• Barrel finish is stainless.
• Finish type is anodized with an overall black color.
• Features a free-floating M-LOK handguard with integral ambidextrous QD cups.
• Includes a complete bolt group assembly and HK buffer and action spring.
• Threading is 1/2 x 28 (suppressor-ready).

Assembled using U.S. and German-sourced components, the MR556 A4 upper kit provides a durable, purpose-built option for shooters seeking the reliability advantages of a piston system. Its combination of HK parts, modular M-LOK interface, and included bolt group make it well suited for precision shooting, training, and professional applications where a robust upper assembly is required.