a .223 from 500 meters is a real stretch for most folks. 350-400 is must more like it.
don't ya know it... it is all i can do to hit a rabbit or prairrie dog at 200yds, so i imagine a large target would be in the realm at 300+...
i guess that's why the remington 7mm mag (and .308) exists. there, you're really reaching out and touching someone.
close in and fast, .45acp / 12ga.
up to 200yds, .223
beyond, 7mm. / .308