Heckler & Koch194197185
H&K 51000385 MR556 Upper Receiver Kit 5.56mm for AR15/M16/M4$2,179.00
Availability: In Stock
Current Stock: Out of stock
The Heckler & Koch MR556 Upper Receiver Kit is a complete 5.56mm AR upper designed to transform a standard AR15, M16, or M4 platform rifle into a proven, piston-driven system. Where direct-impingement rifles funnel hot gases and fouling back into the receiver, the MR556 uses HK's field-tested piston and pusher-rod design—the same technology that powers the HK416 trusted by elite military and law enforcement worldwide. If you're looking to upgrade an existing lower receiver or build a cleaner, more reliable 5.56mm rifle, this kit gives you everything needed to make the conversion.
The MR556 A4 ships as a complete upper receiver and barrel assembly, assembled in the USA from US and German-made components. The system operates reliably with quality 5.56mm or .223 ammunition and delivers the accuracy you'd expect from a professional-grade platform. Because the HK buffer and action spring are shorter than standard mil-spec parts, you'll need a compatible HK lower receiver or installation of mil-spec hammers on non-HK lowers for reliable function—a straightforward upgrade if you're building fresh.
Built for shooters who demand reliability in the field, the MR556 A4 includes practical features: a suppressor-ready barrel with 1/2x28 threads, an adjustable gas regulator tuned to run both suppressed and unsuppressed, and M-LOK handguard slots with integral ambidextrous quick-detach cups for mounting. The extended ambidextrous charging handle, dual-claw design, continuous Picatinny rail interface, and cold hammer-forged barrel round out a kit that prioritizes performance over complexity.
| Product | MR556 Upper Receiver Kit |
| Manufacturer | Heckler & Koch |
| Model Number | 51000384 |
| Caliber | 5.56mm NATO / .223 Remington |
| System Type | Piston-driven (not direct-impingement) |
| Muzzle Thread | 1/2x28 (suppressor-ready) |
| Handguard Interface | M-LOK |
| Rail System | Picatinny (continuous interface) |
| Charging Handle | Extended ambidextrous with dual claws |
| Barrel Type | Cold hammer-forged |
| Gas System | Adjustable gas regulator |
| Assembly Location | USA |
The MR556 uses a shorter HK buffer and action spring that differ from standard mil-spec AR parts. It is fully compatible with HK lower receivers. For use on non-HK lowers, you will need to install mil-spec hammers for reliable function. We recommend verifying compatibility with your specific lower before purchase.
Yes. The MR556 barrel is suppressor-ready with 1/2x28 threads, and the kit includes an adjustable gas regulator specifically tuned to run both suppressed and unsuppressed. You can tune the regulator to your setup and ammunition for optimal performance in either configuration.
The MR556 upper functions reliably with quality 5.56mm NATO and .223 Remington ammunition. Like any piston-driven system, performance and reliability depend on ammunition quality and proper gas regulator adjustment.
This is a complete upper receiver and barrel assembly. You will need a compatible lower receiver to complete the rifle. The kit does not include a lower receiver, buttstock, or grip.
SKU: 194197187 • Width: 13.25in • Height: 4.25in • Depth: 44in • Weight: 11.214oz
Availability: In Stock
Current Stock: Out of stock
The Heckler & Koch MR556 Upper Receiver Kit is a complete 5.56mm AR upper designed to transform a standard AR15, M16, or M4 platform rifle into a proven, piston-driven system. Where direct-impingement rifles funnel hot gases and fouling back into the receiver, the MR556 uses HK's field-tested piston and pusher-rod design—the same technology that powers the HK416 trusted by elite military and law enforcement worldwide. If you're looking to upgrade an existing lower receiver or build a cleaner, more reliable 5.56mm rifle, this kit gives you everything needed to make the conversion.
The MR556 A4 ships as a complete upper receiver and barrel assembly, assembled in the USA from US and German-made components. The system operates reliably with quality 5.56mm or .223 ammunition and delivers the accuracy you'd expect from a professional-grade platform. Because the HK buffer and action spring are shorter than standard mil-spec parts, you'll need a compatible HK lower receiver or installation of mil-spec hammers on non-HK lowers for reliable function—a straightforward upgrade if you're building fresh.
Built for shooters who demand reliability in the field, the MR556 A4 includes practical features: a suppressor-ready barrel with 1/2x28 threads, an adjustable gas regulator tuned to run both suppressed and unsuppressed, and M-LOK handguard slots with integral ambidextrous quick-detach cups for mounting. The extended ambidextrous charging handle, dual-claw design, continuous Picatinny rail interface, and cold hammer-forged barrel round out a kit that prioritizes performance over complexity.
| Product | MR556 Upper Receiver Kit |
| Manufacturer | Heckler & Koch |
| Model Number | 51000384 |
| Caliber | 5.56mm NATO / .223 Remington |
| System Type | Piston-driven (not direct-impingement) |
| Muzzle Thread | 1/2x28 (suppressor-ready) |
| Handguard Interface | M-LOK |
| Rail System | Picatinny (continuous interface) |
| Charging Handle | Extended ambidextrous with dual claws |
| Barrel Type | Cold hammer-forged |
| Gas System | Adjustable gas regulator |
| Assembly Location | USA |
The MR556 uses a shorter HK buffer and action spring that differ from standard mil-spec AR parts. It is fully compatible with HK lower receivers. For use on non-HK lowers, you will need to install mil-spec hammers for reliable function. We recommend verifying compatibility with your specific lower before purchase.
Yes. The MR556 barrel is suppressor-ready with 1/2x28 threads, and the kit includes an adjustable gas regulator specifically tuned to run both suppressed and unsuppressed. You can tune the regulator to your setup and ammunition for optimal performance in either configuration.
The MR556 upper functions reliably with quality 5.56mm NATO and .223 Remington ammunition. Like any piston-driven system, performance and reliability depend on ammunition quality and proper gas regulator adjustment.
This is a complete upper receiver and barrel assembly. You will need a compatible lower receiver to complete the rifle. The kit does not include a lower receiver, buttstock, or grip.
SKU: 194197187 • Width: 13.25in • Height: 4.25in • Depth: 44in • Weight: 11.214oz