I see a lot of guys using 243 for elk too, but not for longer rangesWell, you probably know my choice for mid size big game already. I call the .243, or .243 WSSM depending on the platform a little light for elk sized game or bigger because of the kinetic energy required of larger calibers to smash through heavy bone, retain energy, and remain partially intact enough to still deliver a lethal shot, although one can look on the internet and see some folks that use that round exclusively for elk.
(that being said, I'm going to disagree with myself somewhat)
I'm torn on the line that is drawn by most gun writers that 7mm and above for elk size and bigger, etc. Jim Carmichael killed dozens of elk with a 130 grain in .270 with bullet technology from the 50's and 60's at long range. Why not a 100 grain .243WSSM that runs 400 fps faster?
Now we have bullet manufacturers producing solid copper bullets that will not only violently expand, but will retain over 90% of their weight when recovered from game. So are faster/smaller bullets that are constructed for game and not target, be just as suitable for big game as the larger calibers of old bullet technology? They still have to retain enough energy to smash through heavy bone is the requirement.
There are so many good calibers for elk sized game, as well as calibers for white tail, that is is impossible to name the best.
99% of the lethality of a caliber relies on the shooter and their ability to hit what they are shooting at. That is the bottom line, and you know that.