Heckler & Koch MR556 A4 Pistol Upper, 5.56 NATO, Short-Stroke Piston, CHF Free-Float Barrel, 1/2x28 Thread, M-LOK
This MR A4 Series upper from Heckler & Koch brings the brand's short-stroke gas piston system to the AR-15 platform in a full-size, 5.56 NATO configuration. It features a cold hammer-forged, free-floated steel barrel with a 1/2x28 threaded muzzle and 1:7 twist, a slimline M-LOK handguard, and a continuous Picatinny top rail with white position numbers. German-marked pressure-bearing parts and HK proof marks are present, and the package includes the HK-marked 5.56x45 bolt carrier group, ambidextrous charging handle, buffer, buffer spring, soft case, and MR A4 Series operator's manual.
Condition
Overall Condition: This item is in Excellent condition.
Bore Condition: The bore is bright and clean.
Specific Condition Notes: There is a small scuff on the finish at the end of the handguard from prior handling.
What’s Included
- HK MR A4 Series Operator's Manual
- Buffer spring
- HK-marked 5.56x45 bolt carrier group
- HK buffer with white polymer bumper
- HK ambidextrous charging handle
- HK-branded soft case
Built around HK's short-stroke gas piston, this MR556 A4 upper uses a two-position adjustable gas block with clearly marked "N"/"S" settings. The system is designed for clean operation and consistent cycling, paired with an HK-marked 5.56x45 bolt carrier group included in the sale.
The barrel is steel, cold hammer-forged, free-floated, and threaded 1/2x28. A 1:7 twist rate supports common 5.56 NATO projectile lengths. The threaded muzzle accommodates standard 1/2x28 devices, giving the buyer flexibility for setup.
The slimline free-float M-LOK handguard keeps the front end trim while providing mounting slots where needed. A continuous Picatinny top rail with white position numbers simplifies repeatable accessory placement, and the HK cross-bolt handguard clamp provides a secure interface.
Construction highlights include aluminum receiver and handguard components, an ambidextrous dual-latch charging handle, and German-marked pressure-bearing parts with HK DE, HK shield proof, and CF stamps. This is a recent release within the MR A4 Series and comes with HK's soft case, buffer, buffer spring, and the MR A4 Series operator's manual for a ready-to-configure package.