I'm thinking it's not for shotgun shells. I've looked at, and used, quite a few old reloading tools and can't think of any part in the process where you'd use a tool like this. It looks like it would put a pretty precisely curved edge on a disk or washer. Whatever it must have been meant to form must have been pretty hard, given the force you would get from the threaded shaft and the leverage from the handle. And it needed to be precisely formed, jewelers dome metal disks and washers with a doming block and a round ended punch, which would be enough if you didn't need it to be precise. My first thought was thrust washers, which are both hard and precisely made, but I can't figure why you'd need to hand form them. Just my thoughts....