Ben - I have only fired the 210 Nosler Partition and the 180 Nosler BT with Reloader 22 to find the approximate pressure/velocity levels (one round at one grain intervals). The 210 went 3205 fps with no pressure signs while the 180 BT went 3375 fps with an ejector mark on the case. Accuracy with both bullets was a little less than 1 moa@100 yds. I don't think I can get close to 3400 fps with the 180 BT in my Sendero w/ a 26" barrel, so 3500 fps may be too optimistic for that bullet. But I don't know that for sure. You can find more info at www.accuratereloading.com.
Look under Forums, Big Game Hunting, 338 Ultra? for the data that I posted.
My family and friends all hunt with various 338's and we all use the 210 Nosler Partition for deer and elk. It's penetration was more than adequate on the larger bulls that we have killed, plus, on deer it doesn't seem to do too much damage to the meat - for a 338 caliber bullet. Through the years we have all tried the 250 Nosler on elk and the performance was excellent also. We just prefer the 210's for elk. IMHO on game larger than elk I would bump up to the 250 Nosler in my 340 Wby.
I am interested in what your loads work out to be in your 338 RUM. Please post some info including your fun with antelope this coming weekend. A friend called me this morning and told me that he took a 14 3/4" antelope this past weekend. That is a pretty good antelope for Colorado.
Good luck this weekend