I've never had the pleasure of elk hunting but those with a lot more experience than me say as a general rule of thumb a bullet needs to hit them with 1500ft/lb of energy to cleanly and ethically kill them.You know people kill elk with sharp sticks? I would be willing to bet you any amount of money you want that I could elk all day with a .243 and my 95gr. NBT loads. If you can’t kill an elk with a .243 quit hunting and take up knitting.
243 is under that threshold before 200yards
270 is above that threshold well beyond 400yards.
If you plan on hunting them at the same distance the sharp sticks guys hunt I guess 243 is a viable option.
But it'll never be a 270win. It's like 270 for children.

