H&K 51000385 MR556 Upper Receiver Kit 5.56mm for AR15/M16/M4

The Heckler & Koch MR556 Upper Receiver Kit is a complete piston-driven upper assembly designed to retrofit standard Mil-Spec 5.56mm AR15, M16, and M4 platforms with H&K's proven short-stroke piston system. This is the same operating principle that powers the HK416, trusted by military and law enforcement worldwide. If you're running a direct-impingement rifle and tired of carbon fouling in the receiver, hot gases in your face, and the malfunctions that come with it, this kit offers a genuine upgrade path to a cleaner, more reliable system.

The MR556 A4 upper assembly is assembled in the USA using both US and German-made components, including H&K's proprietary buffer and action spring. The kit ships complete: upper receiver, barrel, forward assist, extended charging handle, free-floating slimline handguard, bolt group assembly, and the HK buffer/spring system. The closed-loop piston design keeps combustion gases out of the receiver entirely, dramatically reducing the wear, fouling, and reliability issues that plague traditional gas-tube rifles. You'll get the same dependable performance and accuracy across a wide range of quality 5.56mm or .223 ammunition.

Important: H&K's proprietary buffer and action spring are shorter than Mil-Spec versions and won't function in fixed-stock AR/M16 rifles. Your lower receiver must be equipped with a Mil-Spec hammer for reliable cycling. If you're building on a standard M4-pattern lower with a carbine buffer tube, this kit will transform your rifle into a piston gun without a complete platform rebuild.

Key Features

• Complete upper receiver and barrel assembly—no additional parts needed to upgrade your lower
• Piston and pusher-rod system eliminates direct-impingement fouling and receiver carbon buildup
• Free-floating slimline handguard for improved accuracy and ergonomics
• H&K proprietary buffer and action spring for optimized piston cycling
• Extended charging handle for easier operation
• Forward assist included
• Assembled in the USA; combines US and German manufacturing
• Compatible with standard 5.56mm and .223 ammunition

Specifications

Frequently Asked Questions

Will this upper receiver kit fit my existing AR15 lower receiver?

Yes, it will mate to a standard AR15-pattern lower receiver. However, H&K's proprietary buffer and action spring are shorter than Mil-Spec versions, so your lower must be equipped with a Mil-Spec hammer for reliable function. It will not work in fixed-stock AR/M16 rifles.

What's the difference between the MR556 piston system and my current direct-impingement rifle?

The MR556 uses a short-stroke piston and pusher rod instead of a gas tube running back into the receiver. This closed system keeps hot combustion gases and carbon fouling out of the receiver area entirely, which reduces malfunctions, extends component life, and keeps your rifle cleaner during and after shooting.

What ammunition does the MR556 upper shoot?

The MR556 functions reliably with a wide variety of quality 5.56mm NATO and .223 Remington ammunition.

Is everything I need to convert my rifle included in this kit?

The kit includes a complete upper receiver assembly, barrel, bolt group, charging handle, forward assist, handguard, and H&K's proprietary buffer and action spring. You will need a compatible lower receiver and a Mil-Spec hammer installed for proper function.