You won't get those velocities using the 338 Excalibur/338-416 Rigby Imp/338-378 Wby Imp/338 Kahn, unless you use extremely long 38"+ barrels, and don't want to shoot your cases more than a few times
Here's a good post Kirby made regarding 338 velocities.
fastest 338 caliber?Build advice?
LTLR has owned them all...he'd be a good reference to query.
Let me suggest the following, since you stated you were "new" to the long range world....In order to obtain the best feedback possible from the forum on your rifle build, start a new thread called "Help me with my new long range build".
Define your build goals in the post by using the following points:
1. Your shooting experience?
2. Current reloading experience?
4. Intended target-Paper/steel or fur or both. If fur list the critters (moose, bear, elk, caribou, deer, sheep, varmints, etc)?
5. Intended distance for the above target?
6. Desired weight (backpacker, horse, atv or truck hunter)?
7. Acceptable barrel life in # rounds before you should expect to re-barrel?
8. Prefer a SAAMI cartridge with off the shelf dies or a Wildcat with custom dies?
9. Do you prefer a Repeater or Single Shot?
10. Do you prefer a custom action or do you have a factory action you want to build from?
There are so many different ways to skin the proverbial cat, that without defining the above, the counsel/advice you receive may be piecemeal and incomplete...by outlining the above, your answers will be focused on your goals and preferences.
Please don't take my post incorrectly....the folks on LRH want you here on the forum, before, during and after your rifle build. But by doing the above you'll allow one central point for all the wisdom that's on the forum to contribute to your build. This forum isn't like others, that the senior smart folks don't have time for questions....great group here.
Good luck, and look forward to hearing about your build and be sure you post up with pics when it's done, and when you harvest something with it!