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The Heckler & Koch MR556A4 upper receiver kit is one of the cleanest ways to move a Mil-Spec 5.56 AR-15 away from direct impingement and onto a piston system without a ground-up custom build. Instead of the standard gas tube, this upper runs HK's short-stroke gas piston and pusher rod, which keeps hot combustion gases and carbon out of the receiver where they normally end up cooking your bolt carrier. That's the same core operating principle HK built the HK416 around, and it's the biggest reason this platform has earned the trust of military and law enforcement users worldwide.
HK reworked this into the A4 configuration, so you're getting an updated barrel profile, an adjustable gas block for tuning suppressed or unsuppressed cycling, a free-floating slimline M-LOK handguard, and an extended charging handle. The heart of it is a 16.5" cold hammer-forged barrel with a 1:7 twist and six lands and grooves, cut for accuracy and built to hold up over a long service life. The barrel is not chrome-lined, which favors precision, and the muzzle is threaded 1/2×28 so you can run your choice of muzzle device or suppressor right out of the box.
This is a complete upper and barrel assembly that drops onto a standard AR-15 lower using regular takedown pins. It ships with the bolt carrier group, extended convertible charging handle, and HK's proprietary buffer and action spring. One thing worth knowing going in: because the HK buffer and action spring are shortened, they're built for a collapsible-stock setup rather than a fixed-stock rifle, and pairing the upper with a Mil-Spec hammer helps keep cycling consistent on non-HK lowers.
Assembled in the USA with US and German-made components, the MR556A4 pulls from many of the same parts developed for the HK416. If you already own a lower you like and want to step up to a proven piston system, this is about as turnkey as it gets. It ships in an HK soft case.
Yes, this is a complete upper and barrel assembly designed to install on standard Mil-Spec AR-15 lower receivers using regular takedown pins. Keep in mind the included HK buffer and action spring are shortened and meant for a collapsible-stock setup, not a fixed-stock rifle. For reliable function on non-HK lowers, HK recommends running a Mil-Spec hammer.
Instead of routing hot gas back into the receiver through a gas tube, the MR556A4 uses a short-stroke piston and pusher rod to cycle the action. That keeps carbon fouling and combustion gases out of the receiver, which cuts down on the buildup that fouls a direct-impingement bolt carrier over a long day of shooting. It's the same operating principle HK uses on the HK416 trusted by military and law enforcement units.
It is. The 16.5" cold hammer-forged barrel comes threaded 1/2×28 for a muzzle device or suppressor, and the adjustable gas block lets you tune the cycling for suppressed or unsuppressed shooting. That flexibility is one of the real advantages of running an adjustable piston system.
No, the MR556 barrel is not chrome-lined, which tends to favor accuracy for precision-minded builds. HK's cold hammer-forged barrels are known for long service life, so you're not giving up durability. It shoots reliably across a wide range of quality 5.56 and .223 ammunition.
| UPC | 642230270213 |
|---|---|
| Manufacturer | |
| Manufacturer Part Number | 51000384 |
| Action | |
| Barrel Length | 16.5" |
| Caliber/Gauge | .223 REM/5.56 NATO |
| Color | Black |
| Model | |
| Package Height | 3.5 |
| Package Width | 13.5 |
| Product Type | Accessory-Uppers |
| Shipping Weight | 11.3 |
| Barrel Finish | STAINLESS |
| Length | 43.9500 |
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