Re: 338 rum (improved???)
You just can't gain enough powder capacity with improving the ultramag case to make it worthwhile. With 338 it takes quite a bit of powder to make a noticable difference. That is why the 338 RUM, 338-300 RUM or edge or whatever, and 338 lapua all shoot exactly the same. The difference between barrels in either one determines whether your accuracy load will be in the 2700's or 2800's fps with the 300 smk and not which of these three you choose. The lapua case has more room for improvement and you can gain some there and jump into the 2900's fps with the 300 grain. Then the huge 378 wby case improved and the 416 rigby imp case can get you over 3100 fps. To get the extra velocities 3-5 grains more powder in an ultramag case just isn't going to matter. 15-20 more grains in the big cases will give you a little extra velocity. The Excalibur which is about the same thing as a 416 rigby improved is about the best you can do with easy to get brass for your mk 5 action. They can get over 3100 fps with the 300 smk in a 30 inch barrel. It is a standard cartridge which makes it much easier.
Predictions are difficult, especially when they involve the future