the bore on 28 ga is .550" 20 is .615" 16 is is .662" 12 is .729" 10 is .775"
Its been awhile, but I'm pretty sure those are correct. 67 ga, should be .411", but over then years it some how became .410"
remember the ga was deteremed by the number of pure lead balls that could be cast from 1 lb of lead. so 12 ga for example you can pore 12 balls of bore diamerter of pure lead. In the olden days they actually did it, now we use constants.
It should be noted that, the bore is measured 9" from the breach face. shot guns barrels are tapered. you litterly roll them on a mandrill.
<<<< knows his shot gun stuff.