I've never done as large a brass case, but I have cobble up rounded, way larger diameter than the case, clamping surface jaws for a vise to reduce the base of some brass. Put a bit of a polished finish on the faces that touch the brass. Then, just do a bunch of light bumps and turns, slow going but doable for a small batch. I'd use a regular sharpie to make a couple or a few opposing lines on the case, should be able to see where it hangs up in the chamber. Anneal, if needed. Just a thought, good luck with it.
