Heckler & Koch MR556A4 Complete Upper Receiver Kit 16.5in 5.56 NATO Piston System

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by Heckler & Koch

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Heckler & Koch MR556A4 Complete Upper Receiver Kit 16.5in 5.56 NATO Piston System

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Retail: $2,179.00

The Heckler & Koch MR556A4 Upper Receiver Kit is an updated complete upper receiver and barrel assembly featuring the A4 improvements, including a new barrel profile, adjustable gas block, and updated handguard. Built to retrofit many Mil-Spec 5.56mm AR-style rifles, it replaces the traditional direct-impingement gas tube with HK’s short-stroke piston and pusher-rod operating system. This closed system helps reduce receiver fouling from hot carbon and waste gases, supporting reliable cycling and consistent performance across a wide range of quality 5.56mm/.223 ammunition. The MR556 A4 upper is assembled in the USA using US- and German-made components and incorporates design elements originally developed for the HK416 platform.

Product Highlights:

  • HK short-stroke piston and pusher-rod system designed to reduce carbon fouling vs. direct-impingement AR-style systems
  • Adjustable gas regulator for suppressed and unsuppressed shooting configurations
  • Suppressor-ready concentric 1/2x28 muzzle threads
  • M-LOK handguard interface with integral ambidextrous quick-detach (QD) sling cups
  • Continuous Picatinny top rail interface between the upper receiver and handguard for optics and accessories

Specs:

  • Brand: Heckler & Koch
  • Model: MR556A4
  • Product Type: Complete Upper Receiver Kit / Upper Receiver Assembly
  • Caliber: 5.56 NATO / .223 Remington
  • Barrel Length: 16.5 in
  • Twist Rate: 1:7
  • Muzzle Threads: 1/2x28
  • Gas System: Short-stroke piston with adjustable gas regulator
  • Handguard Interface: M-LOK
  • Top Rail: Continuous Picatinny
  • MPN: 51000384
  • UPC: 642230270213
  • Assembly: Assembled in USA with US and German-made components

Specifications

Manufacturer:
Heckler & Koch
Category:
upper-receiver
Platforms:
AR-15
Interfaces:
Magpul M-LOKMIL-STD-1913
Length:
16.5"
UPC:
8229128700026
MPN:
51000384
Required Component:
Yes

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Where to Buy

TLO Outdoors
In Stock
$2161.98

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What's Included in this Bundle

Upper Parts Kit
Upper Parts Kit (dust cover, forward assist, etc.)
Barrel
Barrel, 16.5 inch, 5.56 NATO
Handguard or rails
Handguard - M-LOK interface, Picatinny top rail
Bolt Carrier Group
Bolt Carrier Group
Charging Handle
Charging Handle
Bundle Price: $2,198.95

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