The SMR Custom Pre-fit Bartlein Barrel, chambered with an appropriately named B.E.A.R. or Barrel Examination Axis Review chambering process. When you see this logo or name, it guarantees the most accurate and repeatable indication and chambering process on the market, producing the tightest, smoothest, and best shooting chambers available. These custom pre-fit barrels are chambered in 6 BR and are for the Defiance Anti, Anti-X and Ruckus platforms. They have a 1-7.5 twist and 5/8”x24 muzzle thread.