I believe Austin Hogan made this graph of 12ga. guns. For the most part below the 60,000 serial number range, the bores could be in the .750 range. I have a 12ga. lifter that's .745 and a top action that is .756. After that, most 12ga. guns were in the .730 range. I've never measured a 12ga. Parker below .730.
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