O/U shot guns usually are the same gauge but there have been several made that were one barrel shotgun and one rifle, like the The Brno Combo gun 12 gauge shotgun/ choice of .243 Win., .308 Win., or .30-06 Springfield. The Mossberg 12 gauge maverick is the typical O/U shotgun. I used to own a shotgun rifle combo 410/22lr but I forget who made it. I think it was one of those backpack or survival deals.
I have all the Ertl Sportsman figures and I don't recall any of them that came with an O/U shotgun. I parted out all of them the guns are probably in a parts box in storage. I have to go there sometime next week I'll check and see what I have for 1/6th shotguns.