I have never built one but there are the European rimmed cartridges with a diameter very close to a magnum that may be able to be used......9.3x74; 9x57 Mauser.....those types.

I had contemplated a 9.3x74 at one time awhile ago but the 375 Ruger got my attention and was built instead. Still going to build a 9.3x74 but I'm going to do it on a break open single shot at some point.