The Primary Weapons MK1 Mod 1-M Upper is a high-performance 11.85 assembly chambered in 223 Wylde, making it compatible with both 5.56 NATO and .223 Rem. It utilizes the renowned PWS Long Stroke Piston System with a 3-setting adjustable gas regulator optimized for suppressed use. The 11.85 41V50 steel barrel features a 1:8 twist and a black anodized finish. This unit includes an 11.05 M-LOK free-float rail and is designed to fit any standard AR-15 lower receiver, offering the reliability of a piston system with the familiar ergonomics of the AR platform.