H&K MR556 A4 Complete Upper Receiver Kit 5.56mm 16.5″

The Heckler & Koch MR556 A4 upper receiver kit is a complete upper receiver and barrel assembly for AR-15/M16/M4-style platforms. It adapts Mil‑Spec 5.56mm rifles to H&K's piston and pusher-rod system and is assembled in the USA using U.S. and German-made components. The kit includes components intended to provide reliable operation across a broad range of quality 5.56mm/.223 ammunition.

Key Specifications

• The caliber is .223 Rem / 5.56 NATO.
• The barrel measures 16.5 inches.
• The barrel material is steel.
• The barrel finish is stainless.
• The handguard type is a free-floating M-LOK slimline handguard.
• Included is a complete bolt group assembly.
• Included is an HK proprietary buffer and action spring.
• Features include a forward assist.
• Features include an extended charging handle.
• The muzzle is suppressor ready 1/2 x 28.

This MR556 A4 upper kit delivers a factory-configured, piston-driven alternative for AR-style rifles where durability and reduced receiver fouling are priorities. Its combination of a stainless steel barrel, free‑floating M-LOK handguard, and a complete bolt and buffer assembly provides a practical path to integration with compatible Mil‑Spec lower receivers while maintaining straightforward function and serviceability.