diameter is not as important as consistency or thickness (bull barrel or not). The best 3 inner barrels available are PDI 6.04, Edgi 6.00, and PDI 6.01 in that order. Both the edgi barrel and the .04 are bull barrels, meaning they have a thicker outer diameter, therefore more weight and less effected by the shock of piston. For a more budget setup, pick up the Edgi from
here, but if you want the best barrel money can buy, the 6.04 from PDI is what you should get.