Particularly in the pre 1890 guns, people shot black powder and cleaning was haphazard so moisture combined with the black powder residue and created sulfuric acid which corroded barrels. So its common that early guns have been reamed. Then also Parker commonly overbored new barrels, predating the overbore rage of several years ago. As Murphy said, lots of 12's were originally bored at .750. That is about 11ga size, so then is it an 11 or a 12, and the chamber size tells that, but sometimes a gunsmith would bore out a corroded chamber and you get a sloppy fit for a 12 with the delight of blow back upon firing.
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