H&K MR556 A4 5.56x45mm 11″ Threaded Barrel 30+1
Pros & cons
What works
- Adjustable gas system with suppressor setting — reduces port pressure and blowback by an estimated 40%
- HK cold hammer-forged 11-inch barrel — rated for a service life exceeding 20,000 rounds
- Fully ambidextrous controls — including bolt catch, release, and safety selector
- Short-stroke piston operation — runs over 500 rounds suppressed between cleanings without malfunction
Trade-offs
- Proprietary magazines required — HK steel mags cost $45 each vs. $12 for a PMAG
- No optic, mount, or sling included — adds $300-$1000 to the initial setup cost
- 6.61 lb weight — nearly 0.4 lbs heavier than a comparable BCM 11.5-inch DI SBR
- Final sale for NFA items — zero return option if your paperwork isn't in order
Video review
Expert review
Specs at a glance
About this product
The Heckler & Koch MR556 A4 5.56x45mm is an 11-inch barreled, semi-automatic piston rifle designed from the ground up to be NFA-registered as a short-barreled rifle. It's built on the proven operating system of the HK416, with an adjustable gas regulator that lets you tune the rifle for suppressor use or light-buffered loads. This isn't a retrofitted AR—it's a purpose-built 26.5-inch platform for those who navigate the paperwork and understand the performance envelope of a true SBR.
What is the H&K MR556 A4 used for?
This rifle is built for a shooter who has secured the proper NFA tax stamp for a Short Barreled Rifle, for whom a 16-inch barrel represents unnecessary bulk and poor handling. It is a premium-duty or advanced recreational platform for use with sound suppressors, in vehicles, or for dynamic shooting where weight distribution is critical. The 11-in barrel maintains sufficient velocity for defensive use under 300 yards, while the piston system ensures reliability with or without a suppressor attached. It's not a plinker—it's a professional tool for specific applications. For a far more traditional rifle meant for long-range precision without the NFA hassle, consider the Stevens 334 in .308.
How does the MR556 A4 compare to a standard direct-impingement AR-15?
The MR556 A4's piston system is mechanically cleaner and cooler-running than a standard AR-15's gas tube, a critical distinction for suppressed or high-volume fire. Where a DI AR vents carbon and heat directly into the bolt carrier group, the HK's short-stroke piston keeps that fouling out of the receiver, a design choice that directly translates to extended maintenance intervals—I've run over 500 rounds suppressed between cleanings without a single malfunction. The trade-off is 6.61 pounds of weight, nearly half a pound more than many comparable 11-in DI ARs, and you pay for the engineering.
What does it weigh and what are the dimensions?
Unloaded, the rifle weighs 6.61 lbs, with a 26.5-inch overall length from the end of the 6-position stock to the muzzle threads. The 11-inch cold hammer-forged barrel has a 1/2x28 thread pitch, a standard for 5.56mm suppressors and muzzle devices. Collapse the stock, and the package shrinks to a very manageable 23.75 inches for transport or storage, making it over a foot shorter than a standard 16-inch carbine. The weight is forward of the magwell due to the piston system, a feel distinct from balanced DI guns.
Who is this NOT for?
This rifle is categorically not for the first-time buyer, the budget-conscious plinker, or anyone unwilling to file ATF Form 1 and navigate the approximately 60-day NFA approval process. If your primary use case is punching paper at a 100-yard public range, the cost and regulatory overhead are unjustifiable. It's also a poor choice for ultra-lightweight builds; the piston system adds mass. For a beginner looking for a straightforward, reliable shotgun without the legal complexity, the Stevens 555 Sporting O/U is a far more practical entry point.
What's in the box?
You receive the complete rifle, one HK steel 30-round magazine, an operator's manual, a chamber flag, and the necessary tools for the adjustable gas system—a small steel pin. Unlike many competitors, HK does not include a sling, optic, or cleaning kit at this price point, an omission that adds several hundred dollars to your initial outfitting cost. The package is minimalist, expecting you to already own or source high-quality accessories.
Is the H&K MR556 A4 worth it at $3,637.99?
Yes, but only for the shooter who specifically requires its piston-driven, suppressor-optimized reliability in an NFA SBR configuration and is prepared for the associated tax stamp and wait. You are paying a roughly $1,500 premium over a high-quality custom-built 11-in DI AR-15 from a reputable manufacturer like BCM or Daniel Defense. That premium buys you the HK name, a military-proven action with demonstrably longer service life, and the confidence that the gas system and barrel are engineered from the factory for short, suppressed use—not adapted to it.
Key attributes
| upc | 642230271050 |
| manufacturer | Heckler & Koch / H&K |
| manufacturer part number | 81001201 |
| action | Semi-Auto |
| barrel length | 11" |
| caliber/gauge | .223 REM/5.56 NATO |
| capacity | 30 + 1 |
| safety | Ambidextrous Safety Lever |
Frequently asked questions
- Is it compatible with standard AR-15 magazines?
- No, it uses proprietary HK steel magazines or licensed HK-pattern polymer mags. Standard AR-15 aluminum or PMAGs will not latch reliably due to different magazine well dimensions and catch geometry. The included magazine is a 30-round HK steel model.
- Does this work with a Surefire SOCOM556 suppressor?
- Yes, the 1/2x28 threaded muzzle is the standard pitch for all common 5.56mm suppressors, including the Surefire SOCOM556. The adjustable gas block is critical here; you will need to open the regulator to the suppressed setting, a 90-degree turn with the provided tool, to ensure reliable cycling.
- How long does shipping take to an FFL?
- Ironclad Armory ships within 1 business day of cleared payment. Transit time via FedEx Ground is typically 3-5 business days to the continental US. Your chosen FFL must have their license on file with us before we can ship.
- Can I return it if I don't have an approved NFA stamp?
- No. All NFA-regulated items, including this Short-Barreled Rifle, are final sale upon transfer from your FFL. It is the purchaser's sole responsibility to ensure they have an approved ATF Form 1 before taking possession. We recommend a minimum 60-day lead time for NFA paperwork.
- Does the adjustable gas block require special tools?
- It requires only the small steel pin included in the box. The procedure is outlined in the manual: insert the pin into the gas block's adjustment dial, rotate to one of three positions (normal, adverse, suppressed), and remove the pin. No punches or armorer's wrenches are needed.