HK MR556 A4 5.56mm Rifle Review

Heckler & Koch (HK) exploded onto the AR scene in a big way back in 2005 when a Tier 1 SOCOM unit that starts with “D” adopted their better mousetrap short-stroke gas piston operated AR—the HK416. As with most things in the firearms world, this was not an overnight development, linear/direct path, or independent of other design influences. In short, what finally appeared as the HK416 came about because the 1st Special Forces Operational Detachment–Delta (1st SFOD-D) (Delta Force) wanted something harder-hitting than their 9mm HK MP5 and more compact/reliable than their existing M4 carbines. 

Larry Vickers, serving as Small Arms R&D NCO for Delta Force, made a trip to Germany in the 1990s. He was visiting HK’s HQ when conversations arose about improving the US M16/M4. HK’s recent success with remedying the British bullpup service rifle, the SA80, gave it a certain “gravitas” when they offered improvement ideas related to the M16/M4. The thought was to adapt HK’s G36’s short-stroke gas operating system to the M16 family. 

By 2005, the HK416 was in the hands of Delta Force and other SOCOM units such as the Navy SEALs. HK’s success with the HK416 opened the floodgates for other manufacturers to offer designs “improving” the AR platform via piston operation in place of Eugene Stoner’s original gas-impingement design. Proponents of piston-driven ARs point to greater reliability in adverse conditions and less reliance on routine maintenance compared to the direct impingement operating method. Instinctually, gas-piston ARs make sense by the fact that hot gases and powder residue are not dumped into the receiver a la the original direct impingement (DI) design. Anecdotal evidence of bolt carriers being handled soon after long strings of fire and merely wiped down to clean offers a stark contrast to their DI cousins. 

HK gas-piston ARs proved able to operate with barrel lengths below the generally accepted 14-inch barrel threshold of DI AR reliability. Greater dependability with a suppressor mounted is another piston-operated AR advantage with real tactical application. Do not take this as lampooning of gas impingement ARs. The DI rifles are more robust than most give them credit for. However, if your envisioned role involves suppressor use and/or stubby barrels, the short-stroke gas piston-driven AR typified by the HK416 is the way to go. 

While HK has made available HK416 civilian versions, these were limited in quantity/distribution, making them hard to find for even the most die-hard HK connoisseur. However, this trend seems to be ending now that HK has committed to a US operations base involving manufacturing, not just distribution, with a 50,000-plus square foot facility in Columbus, GA.

The latest civilian HK416 version is the MR556 A4, which arrived in November 2024. The emergence of a viable HK-USA entity allows corporate cultures to blend with US market impulses and German HQ mothership qualities. This new attitude is evident in the HK MR556 A4. HK-USA is making certain components in Columbus that combine with German parts to create the MR556 A4. 

HK’s intent with the MR556 A4 was for it to come as close as possible to the select-fire HK416. The MR556 A4 is significantly lighter than its predecessors at 7.5 pounds versus nearly 9 pounds. This is thanks to barrel profile change, forend modification, and different buttstock. The A4’s mag well is chamfered and angled. The A4 is fully ambidextrous (bolt catch/release, magazine release, 45-degree selector levers) with well-done fencing around the levers for a better tactile feel.

There is no tool/pin required to get receiver pins removed with the A4, which was always a head-scratcher with earlier models. Along these lines, the rear takedown pin has a tensioner device that tightens up the upper/lower fit. The new buttstock has an extended castle nut for durability if the rifle is dropped or intentionally slammed down. 

There’s also a QD mount at the end of the A4’s receiver.

The “winter” AR triggerguard has been replaced with a larger radiused version, and the adjustable gas block is well labeled and easily manipulated for shooting suppressed or unsuppressed. Interestingly, the HK MR556 A4’s safety can be activated whether the hammer is released or cocked. This comes from German military requirements and is a worthy feature. 

A new barrel design using non-lined cannon-grade hammer-forged steel replaces the previous chrome-lined mil-spec barrel with the MR556 A4. HK's reasons for this are increased accuracy, performance, and durability. This is based on head-to-head testing of both barrel methods. According to HK reports, “After 12,000 rounds, the HK experienced no degradation in accuracy, while the mil-spec competitor was shooting groups 2.5X larger than its baseline.” That is damn near 1MOA out of a combat rifle. Of special note, HK is putting its money where its mouth is with a lifetime barrel guarantee. 

An enlarged steel barrel nut is utilized in the assembly method of the MR556 A4. The steel barrel nut acts as a heat sink to further reduce temperature build-up in the action area as the rifle heats from use. This, combined with a proprietary HK free-floated M-LOK quad rail design, ensures that accuracy will not be negatively affected by heat from extensive firing or “torque” applied from vertical forward grips or other accessories. The HK short-stroke gas system is self-cleaning in that any excess gas is vented out the front of the handguard just behind the gas block. 

An EOTECH HH II holographic magnifier combo was chosen for use on the HK MR556 A4. Both units together only add 22 ounces to the rifle. The G33 3x magnifier mounts behind the EXPS2 on a tilt mount, allowing for it to be swung out of the way until the situation calls for its use. The G33’s mount provides fast transitioning back and forth from 3x to 1x. Not to drop names, but the G33 3x is the selected magnifier of USSOCOM. Safe to assume it is on many HK416 in the US military inventory. The EOTECH holographic magnifier combination is worthy of consideration by offering a laser holograph reticle, nothing better for fast close-range target acquisition, and quick magnification enhancement as engagement distances increase.

One-and-a-half-inch groups at 100 yards were typical with the rifle wearing the EOTECH red dot and magnifier combo. Ammo used during testing was a combination of loads from SIG SAUER, Black Hills, and Hornady. The MR556 A4 is not a target rifle, which makes those 1.5-inch groups even more impressive. 

Drills quickly moved past “stand and deliver” drills to more dynamic ones involving movement, magazine reloads, and firing from behind cover. I concentrated on using the MR556 A4 in training scenarios involving team tactics and patrolling to contact through Echo Valley Training Center’s (EVTC) 360, MOUT Village, and “Jungle Walk” ranges. A Magpul PMAG D-60 drum magazine was utilized during portions of the T&E, with the thought being that the best reload is no reload. This allowed the HK MR556 A4 to shine in terms of delivering potent firepower. The MR556 A4 recoil impulse is very smooth for a piston gun, rivaling a DI gun, especially the carbine or pistol-length ones. The two-stage trigger on my test rifle had a pull weight of 5 pounds and performed very well.

The MR556 A4 was then put through its paces on different rifle courses that were set up for a pending Origin Shooting 2-Gun practice session—magazine changes, shooting from the non-dominant shoulder, and unconventional positions proved the MR556’s shootability. The total number of rounds fired involving multiple range visits was nearly 600, and the rifle was not cleaned between range visits—after all, that is what short-stroke piston operation is all about. 

The MR556 A4 showed true AR versatility performing within the 100-yard bays at EVTC as well as the HESCO Shoot House. The EOTECH holographic sight and 3x magnifier performed flawlessly, too, as clay targets were sniped with regularity out to 245 yards from the supported prone position. EVTC’s recently added Origin Shooting 3-Gun Rifle Range was also taken advantage of. The range features four different target arrays stretching past 400 yards down a ravine following natural contours.  Variable-sized and shaped steel plates occupy each station. The MR556 A4 was used in the prone, sitting, kneeling, standing, and shooting off of a tripod. The MR556 A4 proved to “hang” well from less well-supported positions and was very manageable in getting into and out of shooting positions. 

HK’s thought process with the original HK416 carried down to the MR556 A4 was to supplement positive AR characteristics with increased reliability via short-stroke gas piston operation. This also allowed for shorter barrel lengths and more confidence when running suppressors. All MR556 A4 features are intended for maximizing effectiveness during a fight or realistic training.

The key component of a fighting rifle is reliability. No matter how accurate or powerful a rifle is, if it does not work 100% of the time, it is a liability. The selection of a fighting rifle is as personal as it gets. The HK MR556 A4 is intended for serious practitioners who plan on using it accordingly. A return to AR basics, such as reasonable weight and great handling with potent firepower, is often the answer for most tactical situations. Heckler & Koch offers such a platform with the MR556 A4. 

CALIBER: 5.56 x 45mm 

ACTION: Semi-automatic 

BARREL: 16.5 inches 

OA LENGTH: 33.25 - 36.5 inches 

WEIGHT: pounds/ounces (empty) 

STOCK: HK416 slimline, six-position stock 

SIGHTS: Optics-ready 

FINISH: Black 

CAPACITY: 10, 30 

MSRP: $3,929 

For more on Heckler & Koch’s new rifle and handgun offering for 2025, please check out HK’s SHOT Show 2025 roundup.