The HK MR556A4 Upper Receiver Kit is an HK-engineered replacement or build component designed around the MR556A4 platform’s 16.5-inch configuration. It provides the upper assembly foundation for reliable AR-pattern operation with HK-spec construction materials.

Key features

• Designed for the MR556A4 platform and 16.5-inch configuration
• Factory-built upper receiver components using aluminum receiver construction
• Steel barrel and gas-system materials for durability and service life
• Upper receiver kit form factor for completing a compatible MR556A4-style build

If you’re building or upgrading an MR556A4-style upper, this HK 51000384 16.5-inch upper receiver kit is a direct platform-matched option.