.223 is an excellent varmint round, but illegal to use on deer sized game in most states. I have often wondered about that.
.223 makes a pretty good deer round if you don't exceed it's capabilities. No shots over 100 yards and good shot placement and it's acceptable.
That is because they restrict cartridges by caliber. They do not want someone using a .22LR (though I have seen deer killed with it, and always one shot). For example, the 64gr PowerPoint load in .223 will work on almost anything a .308 will, and is plenty of cartridge for deer. Bullet construction makes a difference.
To put it in perspective, we are talking differences in diameter of a few hundredths of an inch -- other factors will dominate.
Oh yea that is the high powered so called assault weapons that are to evil and bad for civilians to use.