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Our Rainier Arms Mountain Series upper receiver group was designed for the enthusiast that's looking for a solid foundation for their next upper build using our 1:7 twist Mountain series barrel, our Mil-Spec upper receiver group and Lo Profile gas block. The rest is entirely up to the customer and can be easily built out.
Barrel Material: Chrome Moly Vanadium Steel
Made of Mil Spec M249 Machine Gun barrel steel, with heavy M249 Chrome Lining, (appx. 2 times as thick as an M4 or M16)
Barrel Type: Cold Hammer Forged
Barrel Caliber: 5.56mm NATO Barrel
Barrel Length: 14.5 Mid Gas, 16 Mid Gas, or 18 Rifle Gas
Barrel Profile: Modified Mid Weight Government Profile
Twist Rate: 1:7
Muzzle Thread: 1/2A~—28 TPI (Threads Per Inch)
Diameter At Gas Block: .750
Bore: Chrome Lined
Chamber: 5.56mm NATO
Does NOT come with a BCG, Muzzle device, Charging handle or handguard.
Rainier Arms Mountain Mid Weight Series 1:7 Twist
Rainier Arms Match Lo Pro gas block
Rainier Arms Forged Mil-spec Upper Receiver with UPK
Rainier Arms Barrel nut
Weight: 18" - 2lbs 8.7 oz
Weight: 16" - 2lbs 6 oz