Doesn't look strange to me. Fairly typical late percussion European muzzleloader and it is probably close to 20ga. Most muzzleloader doubles are lighter than we are accustomed in modern break action guns because they lack the extra weight of locking mechanisms, extractors etc and extra metal to accommodate them. Saw somewhat similar one last month in a LA gunshop. Can't compare prices as that one was messed with and had a stock problem.