Heckler & Koch MR556 A4 5.56mm Complete Upper Receiver Kit, 16.5" Barrel, for AR-style Rifles

Overview

The HK MR556A4 16.5" Upper Receiver Kit delivers the unmatched reliability, precision, and engineering excellence that Heckler & Koch is known for. Designed to retrofit Mil-Spec .223 Rem/5.56 NATO AR-style rifles, this complete upper assembly features HK’s legendary short-stroke gas piston system—virtually eliminating carbon fouling and dramatically increasing operational reliability. Built to the same standards as the HK416 used by elite military and law enforcement units, this upper is engineered for shooters who demand top-tier performance.

Key Features

• HK short-stroke gas piston and pusher-rod operating system
• 16.5" cold hammer-forged barrel
• 1:7" twist rate
• Suppressor-ready configuration
• Monolithic M-LOK handguard
• Extended ambidextrous charging handle
• Full-length Picatinny top rail
• Stainless steel barrel finish
• Ships in an HK soft case

Specifications

• Caliber: .223 Rem / 5.56 NATO
• Barrel Length: 16.5 in
• Rate of Twist: 1:7"
• Barrel Finish: Stainless
• Length: 43.95 in
• Product Type: Upper Receiver Kit
• Model: MR556A4
• MPN: 51000384
• UPC: 642230270213

Applications

Ideal for tactical builds, duty carbines, suppressed rifle setups, competition shooting, training, and any AR-15 upgrade requiring elite-level reliability and performance.

Additional Features

• Built to the same durability and performance standards as the HK416
• Designed for superior reliability vs. direct-impingement systems
• Includes HK soft case for secure storage and transport
• Compatible with Mil-Spec AR-15 lower receivers