Actually the .223 round is in the game, it's just called 5.56 round.
Not entirely correct, that was actually a mistake when i first got into reloading. http://en.wikipedia.org/wiki/.223_Remington now mil spec guns designed to shoot 5.56 shoot the .223 rounds no problem whatsoever (one of the reasons i pretty much stick to milspec ARs) but put 5.56 in a .223 the chamber pressure can do some wonky things to the bolt and the castle key.
