If it could be done safely someone would have already done it! You'll also get a good argument from many that a .223 isn't a 300 yard deer gun and really wasn't even designed for deer-sized game! It seems as though someone always wants to use or espouse a small caliber for something it wasn't really meant for and, yes, I know it will kill deer. PS: It would also be best not to get into reloading for the reason you stated, as what you're mentioning would only be advised for someone with many years of experience to even attempt.