I can't imagine a situation that would require a 600+ shot in a SHTF scenario with people shooting back. Ever hear of wait to see the whites of their eyes? It may not be necessary to wait that long now-a-days but in reality 90-95% of shooters can't hit anything out that far, because not that many people pratice long range shooting. Plus the .223 doesn't have a whole lot of ompf out at 600+ yards either. Personally, I would opt for my .300 Win mag for that.
No rifle is the end all rifle, but an AK will take a hell of a lot more abuse than an AR, M1A, or a M1 Garand. That's not an opinion, that is a fact. I have all 3 of those rifles and I like all 3 of them, but in a SHTF life or death situation, I will grab my AK. There will be a lot more times when a close in battle where there will be cover and a need for something to punch through it than there will be for some 600+ yard shot.