Heckler & Koch MR556 Upper Receiver Kit Black

The Heckler & Koch MR556 Upper Receiver Kit gives you a complete path to converting a standard AR-platform rifle into an HK-engineered 5.56 mm system. This black upper assembly swaps into most quality AR-15 builds, letting you leverage H&K's battle-proven pusher rod operating system without starting from scratch. Whether you're upgrading an existing rifle or building a new one around proven HK ergonomics and reliability, this kit includes everything you need for the upper end of your rifle.

H&K's reputation in military and law enforcement circles stems from relentless attention to parts integration and durability—and the MR556 kit reflects that philosophy. The 16.5-inch cold-hammer forged barrel is built for accuracy and longevity, paired with an upper receiver engineered to tight tolerances. The free-floating rail system keeps your handguard isolated from barrel vibration, a detail that matters when precision is the goal. You also get H&K's extended convertible charging handle, which adapts to your preference and handling style, plus H&K's tungsten-filled buffer and proprietary buffer spring that work together to optimize the rifle's firing cycle.

This is a serious upgrade for shooters who want HK's engineering philosophy in their rifle. It's particularly suited to those already committed to the AR platform who recognize that upper receiver quality directly affects reliability, accuracy, and long-term durability. If you've been running a standard AR and want to experience the difference a properly integrated system makes, this kit is a straightforward way in.

Key Features

• Cold-hammer forged 16.5-inch barrel for accuracy and durability
• HK free-floating rail system minimizes barrel vibration and harmonics
• Complete bolt carrier group included and ready to install
• Extended convertible charging handle for ergonomic flexibility
• Tungsten-filled HK proprietary buffer and action spring for optimized function
• Designed to retrofit most high-quality AR-style 5.56 mm platforms

Specifications

Frequently Asked Questions

Will this upper receiver kit fit my AR-15?

The MR556 Upper Receiver Kit is designed to retrofit most high-quality 5.56 mm AR-style rifles. Compatibility depends on your lower receiver and whether it's set up to accept a standard AR upper. We recommend verifying your current lower receiver specifications before ordering, or reach out to our team if you have questions about your specific rifle.

What's included in the kit?

You get a complete upper receiver and barrel assembly (16.5-inch cold-hammer forged barrel with integrated forward assist and free-floating rail system handguard), a complete bolt carrier group, HK's tungsten-filled proprietary buffer, HK proprietary buffer spring, and an extended convertible charging handle. Everything needed for the upper end is in the box.

What makes the HK free-floating rail system different?

A free-floating handguard isolates the rail from the barrel, reducing vibration transfer that can affect accuracy. HK's system is engineered to integrate seamlessly with the MR556 barrel and receiver, maintaining zero shift and supporting consistent performance shot to shot.

Is this kit suitable for long-range or precision shooting?

The cold-hammer forged barrel, tight receiver tolerances, and free-floating rail system are all designed with accuracy in mind. This kit provides solid foundation components for precision work, though final accuracy will also depend on ammunition, trigger quality, optics, and shooter skill.