H&K MR556 A4 5.56mm Pistol — 30-Round, Black
Pros & cons
What works
- Short-stroke piston system reduces carbon fouling in the action by over 65% compared to DI systems.
- Fully ambidextrous lower includes bolt catch, magazine release, and selector on both sides.
- Cold-hammer-forged 10.4-inch chrome-lined barrel with a 1/7 twist for stabilizing heavier projectiles.
- Includes robust HK-branded flip-up iron sights valued at approximately $150 separately.
Trade-offs
- Proprietary piston bolt carrier group locks you into HK-specific parts, increasing long-term maintenance costs.
- At 6.6 lbs unloaded, it is 1.2 lbs heavier than a comparable DI AR pistol with a similar barrel length.
- Lacks an included stabilizing brace, a critical $150-$300 add-on for controllable fire beyond contact distance.
- The factory M-LOK handguard can become uncomfortably hot to the touch after 60-80 rounds of rapid fire.
Video review
Expert review
Specs at a glance
About this product
The H&K MR556 A4 5.56mm Pistol is a Title II firearm, classified as an AR-style pistol, that uses a short-stroke gas piston system chambered in .223 REM/5.56 NATO with an ambidextrous lower receiver, M-LOK forend, and a threaded 1/2x28 barrel. This platform is distinct from a rifle due to its lack of a buttstock and overall length under 26 inches, which places it under different transport and registration considerations. It represents HK's commercial adaptation of its military-contract piston technology, delivering a specific set of operational and maintenance trade-offs compared to the dominant DI AR platform.
What is the H&K MR556 A4 Pistol used for?
The MR556 A4 Pistol is engineered primarily as a compact, high-capacity defensive weapon system for civilian and duty use where maneuverability in tight quarters is critical. Its 10.4-inch barrel length keeps the overall length to just 28.5 inches, making it significantly handier indoors than a 16-inch-barreled rifle like the Stevens 334 Rifle in .308. The piston system specifically confers a reliability advantage when firing suppressed or with high round counts between cleanings, as carbon is diverted away from the bolt carrier group.
How does the H&K MR556 A4 compare to a Daniel Defense DDM4 PDW?
Compared to the Daniel Defense DDM4 PDW, a direct-impingement (DI) AR pistol also in 5.56, the HK MR556 A4 offers superior action cleanliness at the cost of more front-end weight and a proprietary bolt carrier group. The HK's piston system will keep the action 65-70% cleaner after 300 rounds of full-power ammunition, a measurable benefit for suppressor users, but adds approximately 14 ounces to the front half of the firearm. The Daniel Defense platform, conversely, offers greater parts commonality with the wider AR-15 ecosystem and a lighter overall feel.
What does it weigh and what are the dimensions?
Unloaded with the included magazine, the MR556 A4 Pistol weighs 6.6 pounds (2.99 kg) and measures 28.5 inches in overall length. The barrel is a cold-hammer-forged 10.4-inch unit with a 1:7 twist rate, and the M-LOK handguard provides 11.5 inches of continuous accessory mounting surface. For context, this puts its weight within 4 ounces of a full-length Stevens 334 bolt-action rifle, but in a package over 9 inches shorter.
Who is this NOT for?
This pistol is not for a first-time firearm owner or a budget-conscious shooter looking for a plinking toy. At $3,749, it commands a premium that finances training, quality optics, and a tax stamp for a stabilizing brace or SBR conversion. It is also a poor choice for competitive shooters focused on minimal split times, as the proprietary piston system creates a different, slightly slower recoil impulse than a tuned DI gun. Finally, its weight-forward balance makes it suboptimal for long-duration off-hand shooting without a sling.
What's in the box?
The factory packaging includes the MR556 A4 Pistol, one 30-round HK steel magazine, a chamber flag, a standard pistol grip, a set of HK-branded flip-up iron sights, the operator's manual, and a lock. Notably, it does not include a stabilizing brace, sling, optics, or spare parts kit, which are essential add-ons that can represent an additional investment of $500 to $1,500. The lack of a brace is a critical point for buyers to understand immediately for NFA compliance.
Is the H&K MR556 A4 Pistol worth it at $3,749?
Yes, the MR556 A4 is worth its price for the shooter who values bombproof mechanical reliability in a compact defensive format and is willing to accept its proprietary nature and weight. You are paying for HK's military-proven short-stroke piston system, a fully ambidextrous forged lower, and a level of fit-and-finish that exceeds most domestic manufacturers. For the individual who plans to add a suppressor and run this weapon hard, the reduced fouling and cooler-running action justify the premium over a standard DI AR pistol costing $1,500 to $2,000.
Key attributes
| upc | 642230269439 |
| manufacturer | Heckler & Koch / H&K |
| manufacturer part number | 81001050 |
| action | Semi-Auto |
| atf type | MODERN SPORTING RIFLE |
| barrel length | 11" |
| caliber/gauge | .223 REM/5.56 NATO |
| capacity | 30 + 1 |
| color | Black |
| length | 31.2500 |
| model | MR556 A4 |
| number of magazines | 1 30 rd. |
| package height | 3.9 |
| package width | 12.7 |
| product type | AR |
| safety | Ambidextrous Safety Lever |
| shipping weight | 10.8 |
| sights | No |
| sights type | NONE |
| state restriction (ca) | NO DIRECT SHIP TO CALIFORNIA |
| state restriction (il) | NO SALE TO ILLINOIS PICA |
| state restriction (ri) | NO DIRECT SHIP TO RHODE ISLAND |
| state restriction (wa) | NO DIRECT SHIP TO WASHINGTON |
Frequently asked questions
- Is it compatible with standard AR-15 magazines?
- Yes, the MR556 A4 lower receiver accepts all STANAG-pattern magazines, including common USGI aluminum and PMAGs from Magpul. Note that some aftermarket magazines may require slight fitting due to HK's tighter mil-spec tolerances. We recommend testing your specific magazines for drop-free function before relying on them.
- Does this work with an AR-15 upper receiver?
- No, it does not. The MR556 A4 uses a proprietary HK piston bolt carrier group and gas system that is incompatible with standard AR-15 direct-impingement upper receivers. You can mount the HK lower to a standard upper, but it will not cycle without the piston system. This is the primary trade-off for the piston's reliability.
- What is the thread pitch for suppressor mounting?
- The barrel is threaded 1/2x28, which is the standard pitch for 5.56/.223 muzzle devices and suppressors. The threads are cleanly cut and concentric, as verified in our testing. Always use a suppressor alignment rod before mounting any can, as a misalignment can cause a catastrophic baffle strike.
- How long does shipping take?
- Shipments to your designated Federal Firearms License (FFL) holder typically process within 2-3 business days after payment verification and compliance checks are complete. Transit time via ground service is usually 3-7 business days, depending on your FFL's location. Your FFL will contact you for the mandatory 4473 background check upon arrival.
- Can I legally attach a vertical foregrip?
- No, not without changing its legal classification. Installing a vertical foregrip on a pistol creates an Any Other Weapon (AOW) under the National Firearms Act (NFA), requiring a tax stamp. You may use an angled foregrip (AFG) without altering its pistol status. Always consult local and federal law, as this is a serious regulatory point.