Mart, I do have some old Alcan and Fiocchi 12 ga. brass and they measure about .770 inside, more like a 10 bore. The plastic 12 ga. shells measure about .740. I would get the brass shells you intend to use first and determine what size slug/ball is going to be a good fit in those shells. Now we need someone here with more experience on bore guns here to advise on bore size......