The Franklin Armory complete upper receiver comes with everything you will need to install it on your Milspec AR complete lower receiver. Chambered in the amazing .17 Winchester Super Magnum this piston driven upper receiver provides a cleaner and flatter shooting solution for all kinds of pest control. The F17-L R2 has a new Osprey Defense slim piston system. The new piston system automatically adjusts for suppressed or unsuppressed use no adjustments needed! This pairs perfect for use with the 20" barrel which is threaded in the common A 1/2-28 thread pitch perfect for use with compatible suppressors. The new proprietary handguard features an integrated barrier stop / indexing point and is M-Lok compatible. Barrel Length: 20" Threaded A 1/2-28 Thread pitch Handguard/Upper: 15" Integrated Barrier Stop Sights: Optic Ready Twist: 1:9 Charging Handle: Standard Bolt Carrier: F17 Proprietary Gas System: OPS Piston Gas Block: Low Profile Muzzle Device: Aura Color: ODG Caliber: .17 WSM