A WSSM, great idea if it was still being chambered.
Put a 105gr bullet in a 243AI and watch it beat a .243 by 250fps. And with a 80gr bullet it can get 400fps over it. That's more than a little difference. And with less throat damage then it's parent.

http://www.accurateshooter.com/cartridge-guides/243win/

Is a .243 a great round, easy to load with no fireforming...yup, but does a 243AI beat it, yup and in some by ways alot. You won't go wrong either way.