The ARAK-21 Upper Receiver 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 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. Specifications for Faxon Firearms ARAK-21 Complete Upper Receiver: Manufacturer: Faxon Firearms Overall Length: 21.5 in Barrel Profile: Medium Gas System Length: Rifle Length Barrel Twist Rate: 1-8 Handguard Style: Quad Rail Attachment/Mount Type: Picatinny Gun Rail: Quad Rail Gun Type: Rifle Gun Make: AR Platform Gun Model: AR-15 Features of Faxon Firearms ARAK-21 Complete Upper Receiver Ambi Non-Reciprocating Forward Charging Handle Ambi Safety/Selector Bufferless System Quad Rail MuzzLok Muzzle Device Variable-Setting Gas System Self-Contained Firing Pin Retention System Steel Rail System for Bolt Carrier Caliber Conversions Available Package Contents: Faxon Firearms ARAK-21 Complete Upper Receiver ARAK-21 Upper (As Ordered) Shipping Box Manual BAU Removal Tools Bolt(s) And Firing Pin(s) for Ordered Calibers Recoil Spring(s) for Ordered Calibers