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I have a 16" proof research carbon fiber, carbine length, 1:7 twist, threaded .300 blackout barrel with a seekins 5/8"x24 muzzle brake. I bought an assembled custom build to use other parts on my rifle, but I don't shoot 300 and have no desire to start. Barrel appears to have never even been test fired after it was assembled, excellent condition. Price includes shipping with insurance.