The Geissele Super Duty MOD1 16" Heavy Barrel Complete Upper Receiver in Black is a premium heavy-barrel assembly built around a cold hammer forged, chrome-lined 16" barrel with a .750-inch gas journal and 1:7 twist for durable, accurate performance in 5.56 NATO applications. Featuring a mid-length gas system with a Super Compact gas block and a rigid MK16 M-LOK free-float rail, this upper is optics-ready and designed for modular accessory integration. Sold as a complete upper, the 08-627B provides the core components needed for a robust AR-15 build when paired with a compatible lower receiver and lower parts — ideal for precision-oriented, hard-use, or duty configurations.