Faxon Firearms ARAK-21 5.56x45mm NATO 16in Picatinny Upper Receiver 1-8 Twist 1/2x28 MuzzLok Cerakote Midnight Bronze. What came out of that idea is a patented billet-machined monolithic upper receiver (U.S. Patent No. 9032860) that drops onto any standard AR-15 lower and fundamentally changes how the rifle operates. No buffer tube. No gas tube. No forward assist. The ARAK-21 replaces the entire conventional AR gas system with a long-stroke piston with dual spring-loaded operating rods that cycle inline with the barrel. The bolt carrier rides on hardened QPQ steel rails - not aluminum - eliminating metal-on-aluminum wear and delivering AK-pattern reliability with AR-15 ergonomics accuracy and modularity. A non-reciprocating forward charging handle reversible bolt dual ejection ports and an adjustable gas valve make it fully ambidextrous and suppressor-tunable out of the box. Specifications for Faxon Firearms ARAK-21 5.56x45mm NATO 16in Picatinny Upper Receiver: Manufacturer: Faxon Firearms Caliber: 5.56x45mm NATO Barrel Length: 16 in Firearms Threads Per Inch: 1/2x28 Handguard Type: Free-Float Muzzle Device: MuzzLok Finish: Cerakote Color: Midnight Bronze Gun Rail: Picatinny Gun Type: Rifle Gun Make: Faxon Firearms Gun Model: Faxon ARAK-21 Gas System Type: Long-Stroke Piston Magazine Included: No Package Contents: Faxon Firearms ARAK-21 5.56x45mm NATO 16in Picatinny Upper Receiver What firearm models does the manufacturer say Faxon Firearms ARAK-21 5.56x45mm NATO 16in Picatinny Upper Receiver is compatible with? The manufacturer says that Faxon Firearms ARAK-21 5.56x45mm NATO 16in Picatinny Upper Receiver is compatible with Faxon ARAK-21Faxon Firearms URSSB556SSMB: ARAK-21 5.56x45mm NATO 16in Picatinny Upper Receiver 1-8 Twist 1/2x28 MuzzLok Cerakote Midnight Bronze