.308 hands down! It'll give you plenty of options on bullet size, and 500-600 yards is right in its wheelhouse. The .308 is a cheaperround to reload foras well. You wont burn your barrel up shooting the .308 either unlike the higher velocity rounds. You can take that Rem. 30-06 and rebarrel it to .308 but it'll be a long action shooting a short action round, which might be a good thing since you could play with the seating depth a little more because of the magazine being longer in the long action. Something to think about.