My point was that the .223 bullet in question will make a much different sound than "zing" until after it has slowed to below the speed of sound. At that point, you'll be beyond the effective range of the rifle -- the bullet will still be lethal, but unstable.
Sorry, I missed the first part of y'alls conversation, and only saw the post I responded to when I jumped. I'll be more careful next time. I've skipped a lot of posts trying to catch up.