Product Details
The HK MR556A4 upper receiver kit brings HK's piston and pusher rod operating system to a mil-spec AR-15 lower, so an existing rifle can run the same piston design found across the MR556 family. Chambered in 5.56 NATO, which also fires .223 Remington, it carries a 14.50" barrel threaded at the muzzle for a brake, flash hider or suppressor mount.
A 14.50" barrel makes this upper a compact, handy build, but it is under 16.00": installing it on a lower that has a stock creates a short barrel rifle, which must be approved and registered under the NFA before assembly. Check your lower and your paperwork before you put it together.
- Brand: Heckler & Koch
- Model: MR556A4
- Product Type: Upper Receiver Kit
- Caliber: 5.56 NATO / .223 Remington
- Barrel Length: 14.50", Threaded
- Compatibility: Mil-Spec AR-15 Lower Receivers
- Part Number: 51001918
Specifications
| __Model | MR556A4 |
| __Barrel Length | 14.50" |
| __Caliber | 5.56 NATO |
| __Firearm Compatibility | AR-15 |
| __Barrel Type | Threaded |
