When I hunted Red Stag (similar to elk) in Scotland years ago, the gamekeeper at one of the Estates showed me his gun, a Browning bolt action .243.
He told me he'd shot literally 100's of Red Stag over the years and almost every one was a one shot kill.
That goes to acurately placing your round at reasonable distances, on a relaxed target, as an expert shooter.
That being said, although it will, and can, do the job, the .243 is certainly not the optimal, or even reasonably accepted, cartridge for elk for the average shooter or the average Western US hunting conditions.
If you wanted to get extreme, I'm sure you could kill a 1200 lbs bull moose or a 1,000 lbs brown bear with a .22 rimfire, I just wouldn't want to be the idiot trying it! :evil: