Heckler & Koch MR556A1The Firearm Blog

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Built around the legendary HK416 platform, the Heckler & Koch MR556A1 is the newest variant available to the commercial market.  This newest model carries over tried and true features like HK’s short-stroke gas piston system with some new refinements like an M-LOK rail.  Built using a combination of both German and American parts I was curious to see how this new variant performed. H&K was kind enough to send over this review MR556A1 with an EOTech VUDU 1-6 in their new mount.

TFB Review: Heckler & Koch MR556A1

TFB Review: Heckler & Koch MR556A1

Features & Field Strip

TFB Review: Heckler & Koch MR556A1

HK MR556A1, Gen 3 PMAG and Tools

The new MR556A1 now ships with a Gen 3 PMAG, tools, lock, and user manual.

TFB Review: Heckler & Koch MR556A1

HK Scope Mount and the EOTech Vudu 1-6 scope

As mentioned earlier, included with this review rifle was a pre-mounted EOTech VUDU 1-6×24 FFP optic in a new HK branded KDG Modular Optic Mount.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Lower Magwell

The lower receiver features modern refinements like a flared mag well and ambi safety.  However, under the surface, things are a little different than you’d expect.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Lower

Both the front and rear takedown pins feature a second internal detent, and the trigger assembly allows you dry fire the gun, and then put it on safe.  A neat and often overlooked feature.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Stock

One of the trademark features of the MR556 rifles has always been the buttstock.  While it doesn’t feature QD sling attachment points, it does have a sling loop on each side.  I’ll be the first to admit that I really like these stocks and they’re very comfortable.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Handguard Removal

To remove the handguard simply unscrew the bolt at the rear. This exposes the HK short-stroke piston system that’s incredibly simple to take apart for cleaning.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Piston Removal Step 1

With the piston exposed, simply pull backward on the piston to free it from the front gas block.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Piston Removal Step 2

With the piston rod now free simply pull the rod forward to free it from the gas block for cleaning.  I took these photos post review and was astonished at how little carbon had built up on the front of the piston.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Bolt Dissasembly

The bolt carrier on the MR556A1 features a captive takedown pin.  Once this takedown pin has been pushed free, pull up on the release lever to free the firing pin.  Firing pin removed, you can now remove the cam pin and pull the bolt free.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Field Strip

All said and done, the H&K MR556A1 can be fully field stripped for cleaning in a couple minutes with the right knowledge and supplied tools.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Re-Assembly

It’s important to note that you will need the supplied tools to re-assemble the MR556A1.  Both the front and rear takedown pins feature a second internal detent.  Using the supplied tool, you need to press down on the front of the pins before they can be re-inserted.

Accuracy Testing

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Accuracy Testing

First things first, I needed to test the accuracy of the HK MR556A1.  So I took the rifle to my favorite indoor 100yd range with a few different types of match ammo.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Ammo Choices

Four different types of match ammo were used in order to see what the rifle grouped best with.

  • Speer Gold Dot .223 Rem 75 Grain
  • Black Hills .223 Rem 68 Grain Match HP
  • Black Hills 5.56 77 Grain OTM
  • Black Hills 5.56 77 Grain Tipped MatchKing
TFB Review: Heckler & Koch MR556A1

HK MR556A1 Grouping

With the groups fired, and the target returned, there were a couple groups that were staggeringly good.  The clear victor was Black Hills 68 Grain measuring .45 MOA. The worst of the four was Speer Gold Dot 75 Grain measuring 1.19 MOA.  The overall average between the four groups came in at .82 MOA.

At The Range

TFB Review: Heckler & Koch MR556A1

HK MR556A1 At The Range

The base MR556A1 weighs 8.6 pounds, and as pictured the rifle weighs 10.4 pounds (unloaded).  While this weight isn’t ideal for fast transition-based shooting, the gun was remarkably fast on target.  I took the opportunity to run some target drills at Park City Gun Club as was very impressed by how fast I was able to acquire targets in this configuration.

Unlike other short-stroke piston guns, the MR556A1 doesn’t have the same sharp lateral recoil impulse.  This lead to faster follow-up shots that grouped nicely on the two targets I had downrange.  All this paired with the new M-LOK rail makes the rifle very comfortable for offhand shooting.  Even if it is heavy.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Suppressed with Dead Air Nomad-L. Photo Credit: James Rose

Before I sent the rifle back to Heckler & Koch, I wanted to see how well the gun performed suppressed.  Sadly, I didn’t have the H&K recommended OSS can, so I tried the rifle instead with the Dead Air Nomad-L.

Suppressed the gun was incredibly pleasant to shoot with almost no piston-pop.  Even more surprising, the gun maintained an almost perfect 4-o’clock ejection pattern.  I can’t speak for other silencers, but with the Nomad-L attached, the gun was easily hearing safe (and then some) at the ear.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Suppressed. Photo Credit: James Rose

Throughout testing, I used a variety of mags and ammunition to test reliability.  Both suppressed and unsuppressed I never experienced a failure with the MR556A1.  When the dust settled I’d put just under 300 rounds through the gun, and the whole system stayed remarkably clean.

Pros & Cons

The MR556A1 certainly lived up to all the expectations I had for it.  It’s incredibly accurate and smooth shooting.  While proprietary, the short-stroke gas piston system is brilliantly simple and robust.  The rifle performed perfectly suppressed and unsuppressed.  All while using a variety of mags and ammunition.

TFB Review: Heckler & Koch MR556A1

HK MR556A1 Testing

While the MR556A1 did work with the magazines you see pictured it will not work with others.  Mags like Gen 2 PMAGs or Daniel Defense 32 round magazines will not seat in the lower.  Additionally, needing tools to re-attach the upper receiver to the lower is far from ideal.

The Verdict

TFB Review: Heckler & Koch MR556A1

HK MR556A1 with Dead Air Nomad-L

As you might expect, the MR556A1 isn’t cheap and has an MSPR of $3,499.  The rifle uses mostly proprietary parts, is a little heavier than most, and will not accept some legacy magazines.  All that aside, I was very impressed by the MR556A1 in this configuration.  It might not be what I’d recommend for a first-time buyer, but the gun is incredibly well built, functions flawlessly, and is a joy to shoot.

Personally, if I had to pick a piston AR for hard use, this HK MR556A1 would be my first choice.

For more information on the MR556A1 and related products head over to the product page on HK’s website.  Thank you again to Heckler & Koch for sending this over for review.



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