Faxon Firearms AR-15 ARAK-21 Upper Receiver 12.5" Combo 5.56 and 7.62x39

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.
Your ARAK-21 upper receiver will be shipped to you with one “Barrel Assembly Unit" (BAU) in 12.5" Medium profile 5.56 caliber and 12.5" Medium profile 7.62 caliber.
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. Along with your new ARAK-21 upper receiver you will receive your owners manual, and two allen wrenches that will fit the screws that attach the lower forearm, and the front picatinny rails.
Weight: 5 lbs 5 oz
 
Faxon Technical Note on 7.62x39 Caliber Kit:

The factory recommends a high-power hammer spring to use with the 7.62x39 kit. 
Many aftermarket "light" triggers will not reliably ignite 7.62x39 surplus ammunition.  
A spring is included and installed on XRS rifles ordered with the caliber. It is sold seperately for the upper or caliber kit only. 
Faxon offers a Wolff high power spring for sale. 
Faxon recommends HiperFire triggers with a special spring kit for an improved trigger pull. Contact HiperFire for details.
Includes extra bolt for the 7.62x39 barrel