Interesting observation and a surprise to me. Certainly the .357" and .451" bores are not made that much over their nominal dimensions. It should tend to cause terrible leading issues with all the .356" commercial cast bullets out there. It should also tend to cause some underperformance of 9 mm ammo, as the SAAMI test barrel spec is 0.3550" +0.0005" groove diameter, so you know ammunition pressure and velocity testing isn't being done with the loose bores. The only 9 mm I have is European-made .380 and it slugs 0.3553".
I see the complaints about 9 mm bore diameter variability on Castboolits.com. So this leads me to fall back on the standard casting advice to slug your bore before choosing a final sizing diameter.
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