Product Review: The HK416 and the MR556A1

What happens when HK takes its considerable design expertise and applies it to Americaa€™s standard battle carbine, the M4?

Quick, name two letters that, when put together, have become synonymous with accuracy, dependability, and innovation. Heckler and Koch — HK for short — is a German firm that is known for supplying military and police forces with some of the best firearms currently available.

HK rifles have traditionally worked with a system of bolt rollers that lock the action shut. Rifles like the HK G3 and HK 91, using this roller locking system, operate differently than other military rifles such as the Belgian FN FAL and Russian AK-47 that use gas pistons to cycle their actions.

HK has enjoyed tremendous success with the roller locking system but has become more mainstream with its G36 assault rifle. The G36 is a modern rifle constructed largely of polymer but uses a gas piston system that’s more akin to the FAL and AK rather than the roller delayed locking system used in previous HK designs.

On the range, the HK piston gun performed flawlessly with no malfunctions being experienced using a variety of magazines and ammunition.
On the range, the HK piston gun performed flawlessly with no malfunctions being experienced using a variety of magazines and ammunition. (PoliceOne Image)

More recently, Heckler and Koch has applied its considerable experience with weapons research and development to the British SA-80 bullpup. Prior to HK’s troubleshooting and development, the SA-80 had garnered the dubious distinction of being one of the worst battle rifles in modern times. 

Now, with the help of HK, Great Britain’s SA-80 is on par with most other battle rifles currently fielded by the West.

So what happens when HK takes its considerable design expertise and applies it to America’s standard battle carbine, the M4?

HK’s product-improved M4 is dubbed, when dressed in military garb, the HK416 and appears at first glance to be a traditional AR-type carbine. But unlike the conventional AR, the HK416 replaces the dirty and somewhat problematic gas tube with a solid push rod.

This solid rod helps keep carbon and firing residue out of the upper receiver. More information on the gas tube vs. gas piston debate can be found here

There are a number of companies producing piston-driven ARs. I recently had the opportunity to test and evaluate some of these rifles and my thoughts can be read here. 

The HK416 shares many design similarities with piston driven AR’s from other makers but Heckler and Koch has one big leg up on the competition. That leg is military acceptance. Few other makers can claim that their piston AR’s have been thoroughly tested and adopted by military forces.

In fact, the HK416 was originally produced at the request of US special operations troops. These soldiers wanted a rifle that was extremely reliable, required little maintenance, would work in all environmental conditions and function with or without a sound suppressor.

While the HK416 got its start as a rifle for Special Forces, current issuance and usage in the U.S. Special Operations community appears to be very limited. Internet rumors abound about premature parts wear and breakage and many speculate that the HK416 isn’t up to the task.

Others claim that the rifle is too expensive and requires too many unique spare parts, making field repair difficult for troops who may be operating deep behind enemy lines.

Both could be correct, half true or completely false and the truth behind this limited acceptance is not likely to be known anytime soon.

Interestingly, the rifle that started out as a combat carbine for Special Forces soldiers is currently serving with Infantry Marines as the USMC’s new Infantry Automatic Rifle.

While the IAR is fairly new in the Marine Corps inventory, initial reports seem to indicate that the rifle has been well received by those using it.

The HK416 has also had some success internationally and is being used by a number of military and police departments across the globe (the most notable user being Norway, which has been issuing it in large numbers).

In Civilian Clothes
While the HK416 is intended for military and law enforcement sales, the civilian shooter can also purchase his own version of this battle-proven AR. The HK416 in civilian grab is dubbed the MR556A1.

I requested one of these rifles from HK as part of my earlier series on piston driven rifles but a sample was not available in time for my earlier articles.

Like the HK416, the MR556A1 is chambered for the popular 223 Remington and 5.56 NATO cartridges but has no provision for full-auto fire.  Externally, the MR556A1 looks very much like its military counterpart but lacks the chrome plated chamber and bore of the military model.

Many have complained about this lack of plating but the following is taken right from HK’s MR556A1 operator’s manual:

“Like the famous HK416, the MR556A1 uses a German-made barrel produced by Heckler and Koch’s famous cold hammer forging process. The highest quality steel is used in this unique manufacturing process producing a barrel that provides superior accuracy and long service life.

But unlike the HK416, the MR556A1 does not use a chrome-lined barrel. Chrome-lining can sometimes mask bore imperfections and negatively affect accuracy. For the new series of Heckler and Koch semi-automatic MR rifles, HK designers and engineers believe best accuracy comes with an unlined bore.”

The MR556A1 features a 16.5” barrel with a 1x7 barrel twist that’s free floated and covered by a removable quad rail. Housed inside the quad rail is the system’s gas piston and push rod.

These parts are easily field stripped from the rifle but the quad rail must be removed in order to access these parts. The rifle uses a two-stage trigger that’s crisp and clean but a little on the heavy side (roughly seven pounds).

Anyone familiar with ARs should feel right at home with the MR556A1.

It has the potential to be far more reliable than the run of the mill AR. HK engineers did more than just stick a piston in an AR when they built the MR556A1. The rifle uses a much different bolt that’s designed specifically for use in this rifle. Gone are the gas rings and the bolt is longer and more substantial than that used in other AR’s.

The HK’s gas piston parts are heavy and beefy and HK had to increase the height of the top rail in order to house these parts. In the bolt carrier, the cotter key firing pin retainer has been replaced with a captured pin that stays with the carrier. 

Another, more intriguing, change to the carrier is the addition of a firing pin block. This block is basically a lever with a hook on one end that locks the firing pin in place. The hook is latched to the firing pin until the hammer falls.

As the hammer nears the firing pin, the top of the hammer pushes the lever up, releasing the hook and allowing the firing pin to move forward. Using this system, the firing pin is locked until the trigger is pulled.

No more slight firing pin marks when chambering rounds and no more worries about slamfires with out-of-spec primers.

Another interesting change to the rifle’s operating components is a safety lever that can be rotated to safe with the hammer in the forward position. Now the hammer does not have to be cocked to place the weapon on safe and the shooter can initially load his rifle with the safety on before he chambers a round.

On the range, the HK piston gun performed flawlessly with no malfunctions being experienced using a variety of magazines and ammunition.

One thing I would like to make note of is the unique shape of the rifle’s magazine well. The mag well is slightly longer along its front edge and some plastic magazines may not fit. USGI aluminum and, of course, HK steel magazines fit without issues. 

Surefire’s 60 round quad stack magazine also worked fine although the Surefire mag was a little tight in the mag well and wouldn’t drop free when empty.  The unlined barrel seems to live up to HK’s statement that unlined barrels show better accuracy and the MR556A1 shot as well as anyone can expect from a rifle of this quality. 

Quite frankly, anything I can do on the range to test the rifle’s durability and reliability would be nothing compared to what this design has already been through and would be narcissistic, at best. 

Much has also been written on the Internet about the lack of compatibility between HK’s piston upper and lower receivers from other makers. I tried the HK upper on two different lowers and there were no issues with fitment.

While there are some different parts internally, the same is true of all piston AR’s regardless of manufacturer. Spare parts, if needed or desired, can be found at

Complaints? I have two. One is weight. The rifle’s quad rail handguard and heavy profile barrel mean the rifle’s empty weight is well over eight pounds. This puts the MR556A1 in nearly the same weight class as some AR-10 308 rifles.

My second complaint is price; the MR556A1 is substantially more expensive than piston driven AR’s from competing manufacturers. With that noted, there is something to be said about owning a rifle with such an impressive design pedigree.

To quote an infamous TV pitchman, “You know the Germans always make good stuff!”

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