Want to use an AirTac CRBN Upper on your Monk Customs Lower? Look no further! CRBN Uppers are a sleek way to turn any Monk Customs build into a lightweight beast, made with genuine carbon fiber for strength and durability. The Monk Upper features dual hopup windows, fiber-optic back sights, perfect fitment, and updated threads. Plus, a revamped design to match the aesthetics of Monk! All uppers are cross-compatible, so you can swap your tubes and parts between any of your CRBN uppers!  This AirTac Monk HPA CRBN Upper Features: AirTac Monk HPA CRBN Upper: - Revamped framed design for a sleek new look, compatible with Monk Customs Lowers - CNC milled Carbon Fiber and ABS Polymer Threads and Sights - Genuine 3K Twill Carbon Fiber tube - Fiber optic front and rear sights for fast target acquisition - Dual hopup windows for on-the-fly adjustments - Perfected drop-in fitment on a Monk lower with a Maxx hopup - Included Fiber Optics Rods (Green and Red) - Available in Gloss Black, Matte Black, Red, Blue, Purple, Silver, and Gold tube colors Compatible with: - Polarstar Fusion Engine (Drop-In) - F2, Jack, Inferno, N7, Pulsar etc. (Requires CRBN Gearbox) - G&G polymer (Link->https://airtaccustoms.com/products/g-g-cm16-sr-style-polymer-lower)  (SSG1, ARP9). Metal (ARP556) and Lancer Tactical lowers (drop in fitment, has a gap) - Monk Customs lowers (drop in fitment) - (Link->https://airtaccustoms.myshopify.com/collections/maxx) (ATPRO, M4P, M4A) and Retro Arms hopup chambers (drop in) - G&G Stock Hopup, Lancer Tactical Stock Hopup, Prowin Hopup (With modifications) - AirTac Customs  CRBN Gearbox (Link->https://airtaccustoms.com/products/airtac-customs-crbn-gearbox)