Customer brought in an 1881 Marlin he'd recently purchased. Barrel is marked as .40 caliber, and .40-60 rounds can be loaded one at a time and fired, but it is impossible to load the magazine with cartridges. It looks to me as though the receiver may be for a shorter/smaller cartridge and the barrel from another rifle but I've not worked on one of these before. Anyone seen something like this before?

Thanks,