As far as I know, the entry of 12 gauge means that it is chambered for a normal 12 gauge shell. It has no meaning as to the actual bore diameter of the barrel. I don't own a lot of Parkers, but most of the ones I do are from this time period. None of them have regular 12 gauge or 10 gauge actual bores. If the records from that period were listed as actual bore diameters, most of the early guns would be 11 or 9 gauge.
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