Truthfully, I don't know as I don't own a .300RUM. I do however shoot a lot of the 212-X's in a .300WinMag out to 1500yds. They are very accurate (2-3" groups @ 1K) in my current setup which is a 28" Pac-Nor, 9t barrel. Can't remember if it's a 5 or 6 groove that's irrelevant, to this. I'm running them with RL-26 at 3070fps with single digit SD. The only deer I have taken with this bullet was not a real test of it's overall terminal performance or versatility. It was a 100yds, neck shot in the throat. He went straight down and was lying on his legs when I got there. Never kicked.
I took another buck about 4 yrs ago with similar results in reaction. He was approximately 100yds, standing quartering to me. Using same .300WinMag but 208gn A-max. At the shot, he went down but again, no death kicks or stretching or anything. Just stone cold dead. I sat and watched him through the scope for several minutes and kept thinking "he must be playing possum cause he ain't moved a muscle". I waited for him to suddenly revive and hall buggie (as they sometimes do) but he never did. It was like the proverbial "flippin' a light switch".
All that to say; if I were to ever own a LR rig in .300RUM I would shoot the heaviest, highest BC bullet available. That think has a big "gas tank" and personally, I would run it as hard as I could without over pressure (I think I just convinced myself to get one!) to get the performance it promises.
We all know the X is for hunting and the M is for targets but the M, just like the A-max will kill the puddin' out of a whitetail (can't speak for elk).
This past season I shot a nice 8pt at less than 150yds with a 147gn -M from my 6.5-284. It was leaving the muzzle at 3050fps, and it went in at the top of his heart and blew lungs out the back side through a golf ball size exit wound.
I know this is long winded but hopefully it helps you in your decision.