I would consider .308, 30-06, 300 win magazine, or other.
Just a heads up, the "Mag" in ".300 Win Mag." stands for magnum, not magazine. (And that much gun is most definitely not needed for 600 yard shooting)
And to answer your question, if you are dead set on a .30 cal offering, then I say .308 Win. Components for reloading are plentiful, as is factory ammo should you ever want some.
If you are open to other options, then .260 Rem, .243 Win, and .223 Rem also come to mind. Other 6-6.5mm offerings are also great choices.