you might read Bob Brister's "Shotgunning the Art and Science". He details having a barrel bored out taking a 12 ga to maybe an 11 ga and the results. Also you might be able to find a set of chamber mates. Not made by Briley but will take a 12 to 20 without adding much weight at all. They are maybe 4 inches long. Also you might contact John Hosford, he is a wealth of info and can help with what you are contemplating.

This ain't a dress rehearsal , Don't Let the Old Man IN