I'd be cautious of what a shotgun sizer could do for you. I think your chamber is tapered, and it doesn't sound like you have even been able to get to the base which is maybe all the commercial sizer would do.
If you want to go low budget, maybe you could drill a few holes in scrap steel plate, based using some indicating die and measurements of where the case is hanging up in the chamber. I'd bet you could create a series of mild steps to get the brass to chamber, then fire form and trim before loading. Who knows, but I would suspect that the web of a brass case may be hard on conventional shotgun sizing equipment. Only thoughts.
I started thinking about this, too, and realized I was on the wrong track. What puzzles me is that the gun apparently functioned as a 28 gauge shotgun after its original boring out and rechambering. It could be that the thinner brass shotgun shells of the late 19th century "plumped out" more easily at the base when fired in this oversized chamber. But .040 seems like too much for the Magtech to expand.
