What can you hunt with a .223?
Per fish and game codes, is .223 allowed for big game in many states?
As you have a .308, the .243 might be more of a duplicate
What are you trying to do with the new round?
Would you be better off with another .308 in a different configuration?
Longer/ shorter barrel
Lighter or heavier gun
Faster / slower twist for lighter or heavier bullets when reloading?
The velocity delta between .308 and .243 is 400 fps.
How does this impact trajectory / zero at the ranges you intend to shoot?