Along that line, I had a Husky M20, I think it was, that used a 16 bore paper case in the left, smooth barrel and a 16 bore brass case in the right, straight rifled barrel with a 13 bore (actual .705" 13 1/2bore) ball.
The larger bore, if used with a paper or plastic with shot, produced very nicely even cylinder patterns at 25 yards, perfect for rising birds. The smooth bore had an improved modified choke.