Available from Battle Arms Development, the WORKHORSEA(R) Upper Receiver available in both 10.5" and 16" options! These upper receiver sets do not come with a Charging Handle or Bolt Carrier Group to allow for user customization. Barrel Length: 10.5" and 16" Gas System: Carbine Barrel Profile: A2 Barrel Steel: 4150 Chrome-Moly Vanadium Barrel Finish: Nitride Chrome Lining: None Muzzle Thread: 1/2-28 Chamber: 5.56 NATO Twist Rate: 1:7 Barrel Extension: M4 Diameter at Gas Block: .750 Gas Block Type: Low-profile Muzzle Device: A2 Flash Hider Receiver Material: Forged 7075 T6 R Mfg: Battle Arms Development Inc.