Technically any caliber can kill an elk. Depending on shot placement, range, etc. One way to look at is with a .50 cal sidelock muzzleloader. People shoot and kill elk with that at 100 yards or less with a patched round ball. Your muzzle velocity is way less compared to a .243. I wouldn't hesitate to kill an elk with one. Most people think a .270 is under par. I killed a cow elk looking straight at me through the chest blew her heart apart and she still ran 200 yards. (that was with a .270) The only way you can get a elk to drop in their tracks is a spine shot, head, or blow out a shoulder. Just if you use the .243 go with the 100 grain bullet. Either hornady interlock(NO SST) or Nosler Parti.