RE: 338 Ultra-mag why isn't it more popular?
There are a couple of solutions to this dilemma. Since no one really seems to feel the recoil of one of these big bore guns when shooting at big game under field conditions, the real issue is shooting it off the bench.
You can get one of those Benchmaster or Power Isle rests - they contain and absorb most of the gun's recoil and make even a .460 Wby bearable. Buy just one and use it for all your big game rifles.
You can install a muzzle brake, and then remove it for hunting. For a small fee, anyone with basic lathe skills can make you a knurled thread protector to put on when in the field. Only catch is that, at some point, you need to confirm that the gun shoots the same POI with the brake as without.
I also have to agree with Stubble - the .338 RUM has the case capacity to exceed the .340 Wby ballistics - I would be surprised if you couldn't handload it faster.