We wanted to mix up our CRBN uppers in a crazy way - removing the carbon fiber entirely! We did just that, replacing the carbon tube for anodized aluminum, and adding some sleek skeletonized cuts at the front for a super light weight design. The ALU Tube Uppers feature dual hopup windows, fiber-optic front and rear sights, and CNC aluminum parts. These parts are modular, and interchangeable with any of your CRBN Uppers! These uppers are a limited run, so they won't be coming back! CNC Skeletonized Aluminum Tube CNC machined 6061 Aluminum Front Thread CNC machined 6061 Aluminum Front Sight 155mm Inner Barrel Size Strong Nylon Frame Fiber optic front and rear sights for fast target acquisition Dual hopup windows for on-the-fly adjustments Included Fiber Optics Rods (Green and Red) Compatible with: Polarstar Fusion Engine (Drop-In) F2, Jack, Inferno, N7, Pulsar etc. (Requires CRBN Gearbox) G&G polymer (SSG1, ARP9) and Lancer Tactical lowers (drop in fitment) Metal Lower ARP556, Monk Customs and EMG F1 lowers (with modifications) MAXX Hopup Chambers (ATPRO, M4P, M4A), Retro Arms, and Lancer Tactical hopup chambers (drop in) G&G Stock Hopup, Prowin Hopup (With modifications) AirTac Customs CRBN Gearbox