The bullet diameter is never mentioned in these convertibles. .355" to .357". I've not owned, shot, or compared the 2 calibers in a Blackhawk. I would expect a 9 mm to print in a different spot. I do have an OM .45 convertible. .451"-.452". I shoot the Auto cylinder occasionally,, seems anemic after 18–20 grs of 2400 and a 250 XTP.