HK MR556 A4 Upper Receiver Kit - 11"

The HK MR556 A4 Upper Receiver Kit is a complete 5.56×45 NATO upper assembly designed for compatibility with Mil-Spec AR-style lower receivers. This kit allows users to configure an AR-platform rifle with HK’s short-stroke piston and pusher rod operating system, which replaces the traditional direct-impingement gas tube.

By directing propellant gases forward of the receiver, the piston-based system is designed to reduce heat and carbon transfer into the action compared to standard gas tube designs. The upper receiver assembly includes an HK cold hammer forged barrel, adjustable gas block, free-floating rail system, and extended convertible charging handle.

Specifications

• Caliber: 5.56 × 45 mm NATO
• Barrel Length: 11 inches
• Overall Length: 17 inches
• Barrel Type: Cold hammer forged
• Barrel Profile: 6 lands & grooves, right-hand twist
• Twist Rate: 1:7
• Gas System: Adjustable gas block
• Width: 3.07 inches
• Height: 2.68 inches
• Weight: 6.20 lb (complete kit)