Rainier Arms Mountain MOD2 5.56 NATO Barrel 14.5in 1/2x28 Mid Len Black Small. We have brought back our Mountain Series of barrels for those of you who want the accuracy you expect from Rainier Arms in a nearly indestructible package. These barrels are made out of 4150 CMV and nitrided as well. That means these barrels will take anything you can throw at them. They are meant to be shot and shot a lot. The half-fluted profile maximizes heat dissipation and helps reduce weight. Specifications for Rainier Arms AR Mountain MOD2 5.56 NATO Barrel: Manufacturer: Rainier Arms Color: Black Fabric/Material: 4150 CMV Steel Caliber: 5.56x45mm NATO Barrel Profile: M4 Gas Block Journal Size-Diameter: .750 in Threads per Inch: 1/2x28 Barrel Twist Rate: 1-7 Flutes: Yes Gun Make: AR Platform Gun Model: AR-15 Condition: New Package Contents: Rainier Arms Mountain MOD2 5.56 NATO Barrel What firearm models does the manufacturer say Rainier Arms AR Mountain MOD2 5.56 NATO Barrel is compatible with? The manufacturer says that Rainier Arms AR Mountain MOD2 5.56 NATO Barrel is compatible with AR-15Rainier Arms RAB-CMV-556-145-M-HF-BLK: Mountain MOD2 5.56 NATO Barrel 14.5in 1/2x28 Mid Len Black Small