I shoot with a guy who won the top civilian service rifle at Perry a few years ago(missed overall by one point) and he can't hold that with his 40-x in 7mm-08 at 600. (I have seen him go clean with 200-12X and consistently shoot 195s+ with a service rifle in bad conditions at 6.) It is considerably easier to dope the wind with a 7mm-08 or .308 than any load in a .223 (including the 80's which I doubt the Remington will even stabilize. I don't know the twist off the top of my head.) I know you might hold that for maybe a 3 shot group once in a while, but consistently 4"s????? That is some real shooting.
With no wind in Pierre, SD (a very very rare event), we hit groundhogs very infrequently at 600m-700m. But it can be done with good equipment, good training, lots of practice, and God on your side holding the wind back for you.