if you go to the bolt action forums, you'll see there is a thread I started yesterday because I sold my 300 RUM and wanted a 338 RUM.... after doing some "paper hunting balistics examinations" over the internet, I realised, the 338 RUM doesn't give THAT much of a balistic edge over the 338 win mag, so I decided to keep my tikka stainless 338 win mag..... and also holidays are just around the corner so I'll be needing all the cash I can get.
Seriously, I still think the 338 RUM route is much better than the 338 win mag, but since I already own a 338 win, I'll just stick to it until One day I'll have enough money to buy a 338 RUM. The biggest turn on about the Ultra mag is, I think you can download them to 338-06 & 338 win mag levels and if you want more, load them up to the maximum ultra mag levels.