Re: Shooting paper groups out to 600yds with .223
SS 3-9, SS 10x, bushnell 3-12/6-24 ffp, Vortex Viper, nikon 4.5-14/6-18 monarch/buckmasters. If you can stand a lower or fixed power scope the super sniper offerings are probably the best, if you want higher magnification for 300ish the only ones I can recommend are the nikons.
I am in the process of building an AR for similar shooting as you describe and I am going to get the 3-9 super sniper.