That does sound like an interesting design if not the normal approach. I do know after taking many measurement of different bore from 10-16, that it is entirely possible given the extreme difference in 20 and 28s to 10 and twelves. The make concern is that just about no one is producing smaller rifled barrels 'Yet". The reason i say yet is the resurgence in paradox type designs might get some barrel makers think along these lines in this forum. Clearly the pressure is not an issue provided that loads are of shotgun in nature. I would bring a whole new market to otherwise lonely SxSs resting again the closet wall too.
TC