Here is what we measured at the shop..
Factory Entry barrel: I.D. - .720", O.D. - .915"
Factory 18.1 barrel: I.D. cylinder choke at muzzle - .704", I.D. cylinder choke before muzzle where it meets the barrel when screwed in about 2+ inches in - .735", O.D. - .930"
Hope that helps...
Stan