Yep, it's been done (sort of) and called the .338 straight. It didn't work too well, trying to headspace off the mouth of a case that long leads to headspace control problems, and extraction difficulties. The guy at ar15barrels.com does a rimless 357 mag on the .223 case for AR rifles as well, but I have a hard time believing he's getting very good accuracy out of such a setup. I'd like to see one in action.