This might be a little late, but why would you need a larger caliber to shoot 600 yards compared to 500? I shot for years with our state military team shooting rat grade M-16's (5.56/.223) out to 600 yards without any problems. Military matches would shoot 62 grain bullets, but if we were hitting NRA matches, we'd shoot an 80 grain (I think) bullet for less wind play. If your rifle is of decent quality, you should be able to hold MOA at that distance with a .223 just fine. Keep in mind that this is with standard iron sights on these rifles (peep/post).
Anything beyond that, we shot 7.62/.308 and could effectively shoot them out to 1,000 yards with telescopic sights. A good spotter could hold you under MOA at that distance without any problems. This is with the Remington 700-based sniper rifle format.
Try the .223 out at that range. You might be pleasantly surprised!