The Geissele Super Duty MOD1 .300 BLK 8" 1:7" CHF Chrome Lined Bbl DDC 08-553S upper is purpose-built for suppressed, short-barreled applications where .300 Blackout does its best work. It ships as a genuinely complete assembly, BCG, charging handle, and muzzle device all installed, so a compatible registered SBR or pistol lower is close to the only thing standing between this and a finished build. The 8" barrel is cold hammer forged and chrome lined with a 1:7 twist, stabilizing both subsonic and supersonic .300 Blackout loads without needing a barrel swap between the two. What really sets this Geissele Super Duty MOD1 8 upper apart is the gas system: instead of a standard carbine-length setup, Geissele runs its patented Phased Array gas system with the Tri-Port design, tuned to a Custom Pistol length specifically for this barrel. That precise gas metering matters even more in a short-barreled, suppressed .300 BLK build than it does on a standard 5.56 carbine, where timing gets a lot less forgiving. A 7" Super Modular Rail MK16 mates to the upper through Geissele's center aligning tab, keeping the whole package compact without sacrificing M-LOK real estate for lights or grips. Cycling runs through Geissele's Nanoweapon-coated Reliability Enhanced BCG, and the HUXWRX Flash Hider-QD muzzle device threads on at 5/8x24, the correct pitch for .300 Blackout muzzle devices and suppressors rather than the 1/2x28 used on 5.56 builds. At approximately 3.3 lbs and finished in Desert Dirt Color, this Geissele upper receiver is ready to drop onto a compliant lower. Key Features Ships complete with BCG, charging handle, and muzzle device installed 8" cold hammer forged, chrome-lined barrel, 1:7 twist Patented Phased Array gas system with Tri-Port design Custom Pistol length gas configuration for reliable suppressed cycling 7" Super Modular Rail MK16 via center aligning tab 5/8x24 muzzle thread for .300 Blackout compatibility Nanoweapon Reliability Enhanced BCG with Stressproof Bolt Weighs approximately 3.3 lbs, finished in Desert Dirt Color Geissele Automatics Geissele Super Duty MOD1 .300 BLK 8" 1:8" CHF Chrome Lined Bbl DDC Complete Upper Receiver Group 08-553S