You wouldn't need to worry about the case head diameter difference between the 9mm and the 223. You'd be using a 223 bolt face which matches the rim diameter of the brass. A custom reamer from Dave Kiff and you're all set. It would be a fun plinker, but I don't think you'd find any bullets suitable for hunting. I'd run it out the extra .003 and make it a .358 based case where you would have a good selection of bullets out to 300+ grains. I haven't looked, but I'd bet it's already in Dave's list.