H&K MR556 A4 Complete Upper Receiver Kit 5.56mm 16.5″

SKU: BILL|HK 51000384
SKU: BILL|HK 51000384 Category: Tag: Brand:

$2,328.99

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The Heckler & Koch MR556 A4 upper receiver kit is a complete upper receiver and barrel assembly for AR-15/M16/M4-style platforms. It adapts Mil‑Spec 5.56mm rifles to H&K's piston and pusher-rod system and is assembled in the USA using U.S. and German-made components. The kit includes components intended to provide reliable operation across a broad range of quality 5.56mm/.223 ammunition.

Key Specifications

  • The caliber is .223 Rem / 5.56 NATO.
  • The barrel measures 16.5 inches.
  • The barrel material is steel.
  • The barrel finish is stainless.
  • The handguard type is a free-floating M-LOK slimline handguard.
  • Included is a complete bolt group assembly.
  • Included is an HK proprietary buffer and action spring.
  • Features include a forward assist.
  • Features include an extended charging handle.
  • The muzzle is suppressor ready 1/2 x 28.

This MR556 A4 upper kit delivers a factory-configured, piston-driven alternative for AR-style rifles where durability and reduced receiver fouling are priorities. Its combination of a stainless steel barrel, free‑floating M-LOK handguard, and a complete bolt and buffer assembly provides a practical path to integration with compatible Mil‑Spec lower receivers while maintaining straightforward function and serviceability.

Additional information

upc

642230270213

Manufacturer

Heckler & Koch / H&K

Manufacturer Part Number

51000384

Action

.223 REM/5.56 NATO

Barrel Length

16.5"

Caliber/Gauge

.223 REM/5.56 NATO

color

Black

model

MR556 A4 Upper Receiver

Package Height

3.5

Package Width

13.5

Product Type

Accessory-Uppers

Rate of Twist

1-in-7"

Shipping Weight

11.3

Barrel Finish

Stainless

Length

43.9500

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