- Short-Stroke Gas Piston Operating System
- Cold Hammer-Forged 5.56 NATO Barrel, 1:7 Twist
- Free-Float M-LOK Handguard
- Adjustable Gas Regulator, Suppressor-Ready
- Includes HK BCG, Buffer, Buffer Spring, and Ambi Charging Handle
- Fits Mil-Spec AR-15 Lowers
- Color: Matte Black
- Caliber: .223/5.56
- MPN: 51000384
The Heckler & Koch MR556A4 Upper Receiver Kit converts any mil-spec AR-15 lower into HK's renowned short-stroke gas piston operating system. The 16.5-inch cold hammer-forged barrel is chambered in 5.56 NATO with a 1:7 twist, and the adjustable gas regulator makes the upper fully suppressor-ready. The slim free-float M-LOK handguard features a full-length Picatinny top rail, and the kit ships with an HK bolt carrier group, heavy buffer, buffer spring, and extended ambidextrous charging handle. Hot carbon fouling and waste gasses are kept out of the receiver, dramatically reducing maintenance requirements and improving reliability.
