How about punching a 458 magnum to straight wall case. That would give an outside diameter at the mouth of .513. It would require inside reaming of the neck for the .500" bullet and it would be a thin neck but it would give you a lot more powder space.....
Probably not as workable as one would like and you should look for a case with .013 to .015 (OD=.526-.530) wall thickness instead of .0065 (OD=.513) that the magnum cartridge will provide.