The ARAK-21 is a gas piston actuated, complete upper receiver assembly designed to seamlessly interface with the standard AR-15 platform. The total extent of the interface is simply the take down pin, and the pivot pin. The new ARAK-21 upper receiver offers you all its features with no change to your current lower receiver and can be installed in less than one minute with no tools. The BAU is comprised of the barrel, gas block, muzzle brake, gas piston, gas adjuster, and barrel trunnion. The only removable part of this assembly is the gas adjuster knob. The gas adjuster knob has 4 settings. Three of the settings allow different gas flows for optimum operation, and one setting blocks the gas flow for a single shot operation. The upper receiver includes the charging block, standard charging handle, lower forearm with screws, three aluminum full length picatinny rails, three insulating shims, bolt carrier assembly, lower recoil lug with spring guide rod, and dual recoil spring system. Due to the unique and innovative features of the ARAK-21 upper receiver, there are very few parts that are interchangeable with a standard AR-15. The firing pin, extractor spring, and ejector spring are the only parts that are standard and interchangeable with an AR-15 upper receiver.