Interesting you should mention replacing the .223 with the .243. Knight's Armament is currently working on a "SPR" variant that uses a 6mm short cartridge with ballistics similar to the .243, which resolves the low knockdown-power and long-range deficiencies of the 5.56x45 NATO. As I understand it, prototypes are already in service in Afghanistan.
GMTA! $;-)
What does that 6mm case look like? Long and thin, or short and fat? I know "short and fat is where it's at" in bolt hunting and sniping rifles, but in an autoloader, case geometry is optimized by many other factors.
I know Armelite makes a .243 AR-10, that would be fun to shoot, but not if it's an orphan. I'd love to see a class of .243/6mm military rifles! The Brits almost adapted a 6mm in the 1950s, but lost out to our 7.62NATO.