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Heckler & Koch

HK MR556 A4 16.5"

HK's commercial semi-auto HK416. Short-stroke gas piston with adjustable regulator, ambi controls, M-LOK free-float rail.

Key Highlights

  • Civilian MR-series update derived from HK416 architecture
  • Short-stroke gas piston eliminates carbon fouling in the receiver
  • Adjustable gas regulator tunes for suppressed or unsuppressed fire

Specifications

Price
$3,999
Weight
7 lb 11 oz
Calibers
5.56x45mm
Rail Type
mlok

Capability Profile

0-10 Scale Across 8 Axes

Compatibility Tags

family: modern-riflefamily: ar-patterntrigger: ar-fcginterface: picatinny-tophandguard: mlokbarrel: longthread: 1/2x28mag-type: stanag

Overview

The MR556 A4 is HK's current commercial MR-series update derived from HK416 architecture. The A4 brings fully ambidextrous controls, a slimmer M-LOK free-float rail, and reduced weight versus the legacy MR556A1. The short-stroke piston system runs cooler and cleaner than direct impingement, and the 2-position adjustable gas block lets you tune the action for suppressed or unsuppressed shooting without changing parts. This is the gold-standard piston AR if budget is not the constraint.

Key Features

  • 16.5" cold hammer-forged barrel, 1:7 twist, 1/2x28 threads
  • Short-stroke gas piston operating system (HK416 pattern)
  • 2-position adjustable gas block for suppressed or unsuppressed use
  • Fully ambidextrous controls (safety, mag release, bolt catch)
  • HK Free Floating M-LOK Rail System with full-length top Picatinny
  • Tungsten-filled HK proprietary buffer and recoil spring
  • Extended convertible charging handle
  • Accepts standard STANAG / AR-15 magazines

Pros

  • +Civilian HK416 — proven combat-spec gas piston system
  • +Adjustable gas regulator is rare at any price point
  • +Cleaner running than DI, especially when suppressed
  • +Excellent QC and tight tolerances out of the box
  • +Ambidextrous controls work cleanly (no afterthought conversions)
  • +STANAG mag compatibility and AR-pattern lower controls

Cons

  • MSRP $3,999 puts it well above premium DI ARs
  • Heavier than comparable DI rifles (7.67 lbs unloaded)
  • Proprietary barrel, BCG, gas system, and handguard (no DI parts swap)
  • Limited aftermarket beyond HK and Geissele (who supplies HK)

Detailed Specifications

caliber
5.56 NATO
Barrel Length
16.5"
Overall Length
33.25" - 36.5"
weight
7.67 lbs with empty 30-round magazine
Gas System
Short-stroke gas piston with 2-position adjustable gas block
railSystem
Slimline M-LOK handguard with 20.3" Picatinny top rail
Trigger
Two-stage, 4.5-5.6 lb approximate
Twist Rate
1:7 right-hand, 6 lands & grooves
threadPitch
1/2x28
msrp
$3,999

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