HK MR556 A4 Upper Receiver Kit 5.56 NATO 11.5″

Description

This MR556 A4 complete upper receiver kit from Heckler & Koch provides a direct replacement upper/barrel assembly for Mil-Spec AR-15/M16/M4 platforms to use HK’s piston and pusher rod operating system. The closed piston system keeps combustion byproducts out of the receiver area, promoting reliable function across a wide variety of 5.56mm/.223 Rem ammunition. The A4 update includes a revised barrel profile, adjustable gas block and a free-floating slimline handguard; the kit ships with the bolt assembly, HK proprietary buffer and action spring in an HK soft case. Note important compatibility considerations in the specifications below to ensure correct function with non-HK lowers.

Key Specifications

• The caliber is .223 REM/5.56 NATO.
• The barrel length is 11.5.
• The barrel finish is STAINLESS.
• The barrel material is STEEL.
• The rate of twist is 1-in-7″.
• The receiver and components have a Anodized finish.
• The kit fitment is specified as AR-15.
• The listed color is Black.
• The kit includes Bolt and Buffer as part of the assembly.
• Other notable feature: FORWARD ASSIST.

This MR556 A4 upper is built for shooters who require a robust, piston-driven replacement upper with HK-derived engineering and practical features like an adjustable gas block and free-floating handguard. It’s a solid choice for use in duty-style or high-reliability AR-platform configurations where reduced receiver contamination and proven componentry are priorities.