Another vote for the M77-MKII, in 30.06.

What would you be doing with a quad rail, on a 600 yard gun? IMO, you are
trying to do way too much with one rifle. Figure out the attributes you want most.
Do you really want to put a bunch of tacti-crap on a hunting-rifle, even if you
did go with an AR? Do you have situations where you will be trying to shoot
a deer at over 300 yards?