Heckler & Koch MR556 A4 – 5.56 NATO, 16.5" Barrel, 30-Round Magazine (81000827)
The MR556 A4 is HK’s commercial short-stroke piston rifle in 5.56 x 45 mm NATO, updated with fully ambidextrous controls, a slimline M-Lok handguard, and a two-position adjustable gas block for suppressed or unsuppressed use. This listing is the 16.5-inch barrel rifle that ships with one 30-round magazine. HK part number 81000827, UPC 642230269408. This listing is for the MR556 A4 16.5" / 1x30rd configuration only.
Short-Stroke Piston, Tunable Gas
HK’s gas-operated short-stroke piston keeps propellant fouling out of the action compared with direct-impingement ARs. The adjustable gas block is set for suppressed or unsuppressed shooting so you can tune the cycle without rebuilding the gun. Barrel thread pitch is 1/2 x 28.
Ambidextrous Lower and Slim Furniture
- Fully ambidextrous bolt catch/release, magazine release, and 45° safety/selector levers.
- Ambidextrous charging handle.
- Lightweight M-Lok handguard with QD sling cups left and right.
- Adjustable buttstock — 6 positions, 96 mm of travel per HK’s operator manual.
- STANAG-compatible magazine feed; this SKU includes one 30-round magazine.
Barrel and Accuracy Confidence
HK lists a 16.5-inch barrel with 6 lands & grooves, right-hand 1:7" twist, and has published a lifetime barrel bore guarantee for the MR A4 series. Trigger pull is approximately 4.5–5.6 lb.
Heckler & Koch MR556 A4 Specifications
| Manufacturer | Heckler & Koch |
| Model | MR556 A4 |
| Part Number | 81000827 |
| UPC | 642230269408 |
| Caliber | 5.56 x 45 mm NATO |
| Operating System | Gas operated, short-stroke piston |
| Mode of Fire | Semi-automatic |
| Barrel Length | 16.5" |
| Twist | 1:7" RH, 6 lands & grooves |
| Barrel Thread | 1/2 x 28 |
| Overall Length | 33.25" (stock retracted) – 36.5" (stock extended) |
| Height | 7.5" |
| Width | 3.07" |
| Weight | Approx. 7.67 lb (HK operator manual: with empty 30rd magazine) |
| Magazine | One 30-round included; STANAG compatible |
| Trigger Pull | Approx. 4.5–5.6 lb |
| Buttstock | 6-position adjustable, 96 mm travel |
| Handguard | Lightweight M-Lok, slimline |
| Gas Block | 2-position adjustable (suppressed / unsuppressed) |
| Controls | Fully ambidextrous lower; ambi charging handle |
Frequently Asked Questions
What magazine ships with 81000827?
HK lists this ID as MR556 A4, 16.5" barrel, 1-30rd magazine.
Is the gas block adjustable for a suppressor?
Yes. HK publishes a two-position adjustable gas regulator for suppressed or unsuppressed use.
What is the twist rate?
1 in 7 inches, right-hand, 6 lands and grooves.
Ambidextrous controls?
Yes — bolt catch/release, magazine release, 45° selector, and charging handle are published as ambidextrous on the A4.
Internal Reference:
81000827
Barcode:
642230269408
| Brand: H&K |
| Model: MR556 A4 |
| Caliber: 5.56mm |
| Capacity: 30 |
| Firearm Action: Semi-auto |
| Barrel Length: 16" - 36", 17" |
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This product may have shipping restrictions in some states.
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This item cannot be shipped to the following locations due to legal or carrier restrictions:
Restriction Details
This product cannot be sold or shipped to your location due to magazine capacity restrictions. Magazine capacity cannot exceed 10rd. If you believe this to be incorrect or if you're exempt from the restriction, please contact us at 636-791-1655.
Affected States/Territories (10):
Restriction Details
Colorado prohibits magazines with a capacity exceeding 15rd. If you believe this is in error or if you are exempt from the restriction, please contact us at 636-791-1655.
Affected States/Territories (1):
Restriction Details
Vermont restricts rifle magazine capacity to 10rd. This exceeds the state's restricted capacity. If you believe this is in error or if you are exempt from the restriction, please contact us at 636-791-1655.