Many shotuns built onsurplus Mauser actions over the years, often for European trade, and often 20 or 28 bore. Not uncommon at all. At least one importer/wholesaler (SARCO) has 28 bore barrels for Mauser actioned shotguns.
But I have a couple of old J.C. Higgens Bolt shotguns
Be careful with those.I don't have the model numbers on hand, but Sears had a recall on some of these about 15 years ago. Had problems with the bolt breaking and blowing back, if I remember correctly. Do a search on JC Higgins shotgun recalls, and you should be able to find model numbers.