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The groove diameter is probably around .535 or more, with a .520 bore. These will require a bullet with at least a thousands or two more than groove diameter to shoot well.They take either a .510" or .515" bullet. Originals were .515" but reproductions are likely .510" bore.
Neat guns. I have an original that was converted to 50-70 after the Civil War. Sadly here in PA we can use them in regular rifle season but not in our early ML season (doe only).