No not really..... .223 = 5.56mm a 243 = 6mm, you do the math... with fmj ammo the 243 isnt gonna leave any bigger of a hole then a 223. besides the 5.56mm is known to tumble upon impact with flesh and bone I believe with the higher velocity and heavier bullet of the 243 makes for a much more stable projectile and it would just pass through causing far less tissue damage then what the 5.56mm is known to do.

something else to consider is with a service rifle the idea is to keep the lead flying. a 243 would over heat a barrel way to fast in a fire fight causing severe damage to the weapons barrel. I guarentee our armed forces really dont want to have to change the barrels of there service weapons after every fire fight.

IMO there is really nothing wrong with the 5.56mm. instead of worrying about the caliber our military should worry more about the platform it is fired from.