I've had much debate with a few folks I hunt with over the last couple years about this very topic, mostly because of kids we're trying to shepherd into elk hunting. I grew up in an undergunned household, by today's elk-hunting standard, and my first elk hunts I carried a 257 with 100gr PT's at at book speed of 2830. I doubt they even got that. I didn't get elk with that gun, but my dad did.
I know a 243 can easily get beyond that 2830 with 100gr Partitions. I also know that a 6mm 100gr Partition is great elk medicine when good hunting and good shooting are part of the equation. I also don't think that handicapping youngsters with something that leaves little room for error when it comes to shot angles/placement/distance is a good idea.
Certainly, in the hands of someone who knows what elk fever feels like and can handle the pressure and can pull off a good shot during less than ideal circumstances, a 243/6mm with appropriate bullets is just fine for elk hunting, or more to the point, elk killing and recovery. I don't like handicapping kids though with the same setup.
We've not seen elk during the hunting season the last 2 years, so there have been no successes or failures to note, but I have convinced a couple folks to move to a 308 and 150gr bullets for their youngsters' hunting when elk is in mix. There are other steps between a 243 and 308 that I think would have been just fine, but other circumstances came into play, and 308's were chosen.
Handloading and bullet casting have been very handy in these circumstances for getting the kids started with very reduced loads so they get much trigger time and get comfortable with a rifle while dealing with vastly reduced recoil and noise. One youngster who will be 14 this year, and a small kid, has graduated to 308 nicely, and we've been working him toward 150 gr bullets, adjusting bullet weight, powder charge, velocity with each step. He has a knack for shooting, and loves it, so it hasn't been a chore to help him progress. So fun to watch! Excited at the prospect of doing this same process for my own in the future.
Bringing this back to topic: 243 is plenty good and ethical in the hands of a knowledgeable hunter and shooter when elk are being pursued. It is a handicap for those who are more green, and IMO, it is a ticket to a rodeo. Far better to give a youngster a gun with more bullet weight, momentum, and surface area, as long as they can shoot it proficiently.
