I may be wrong but I think it was the first of the big beltless magnums off the 404 Jeffery case.The case has a longer shoulder than the Remington 338 ultra magnum and although both are beltless they cannot be swapped. The 338 imperial is a great round with more water capacity than a 338 ultramag.Imperial is now ??defunct and brass may be hard to come by. If I recall they had brass quality issues and only a limited number of firings with one shell was possible.You may be able to form brass by necking down a Remington 375 ultra mag.

This round can fire a 250 gr bullet at 3000 fps vs a 338 WM at 2550 fps, impressive. The recoil is of course--noticable.

Hope I haven't confused things.