Heckler & Koch MR556 A4 Complete AR-15 Upper Receiver - 5.56 - 11"

The Heckler & Koch MR556 A4 Complete AR-15 Upper Receiver at 11" brings HK's short-stroke gas piston system to a compact platform, retrofitting a standard AR-style rifle with the same operating principle used in the HK416.

Features:

• 11" cold hammer forged barrel chambered in 5.56 NATO, 1:7 twist
• HK short-stroke gas piston operating system
• Adjustable gas block for suppressed or unsuppressed use
• HK Free Floating Rail System handguard
• Complete MR556 bolt carrier group included
• Extended convertible charging handle
• HK proprietary tungsten-filled buffer and buffer spring
• HK-branded soft case included

Piston operation keeps this compact build running cleaner than a direct-impingement system would at the same barrel length, where fouling tends to accumulate faster, and the HK Free Floating Rail System handguard reduces barrel deflection while giving you a stable mounting platform for optics and accessories.

Because this 11" barrel measures under 16", this complete upper is intended only for an AR pistol or a legally registered short-barreled rifle — attaching it to a standard rifle lower without the correct configuration violates federal law.

This complete kit installs on any standard AR-style rifle receiver, giving you a plug-and-play upgrade path to piston operation in a compact package.

Heckler & Koch has supplied duty and military weapon systems worldwide for decades, and the MR556 brings HK416-derived piston technology to the civilian AR-15 market.