If the farthest you can realistically shoot is 500 yards then I would just stick with .223/5.56. It will be more than adequate at those distances and being cheaper to shoot definitely doesn't hurt.
Yeah, but I still kind of want a 308 anyway. Even if not practical, will be more fun.
I've not even properly sighted in my AR anyhow. I just completed the build a month or so ago. Because of my work schedule I could never get to the LGS in time to get ammo there, so was going to Wal-Mart after dark to get ammo. All they had there was $35, 20rd boxes of match grade Winchester .223. Due to the price I only bought 60 rounds there and used those to break the gun in. Shoot a few rounds, clean. Shoot a few rounds, clean. After that I said fuck it and just started shooting. Finally got to the LGS a couple weekends ago and they had $8, 20rd boxes of PMC x-tac 5.56 so got some of those. I'll get it sighted in once I go to a proper range and aren't just shooting logs in the woods.