Trigger.
Barrel and trigger. Those two things are what's gonna determine your accuracy - well, besides you. 600 yards isn't long range, either, even for hunting these days.
If you want one that will always get the job done, perform exellently for accuracy, and will fit all future needs, then look at this one:
http://geissele.com/hi-speed-national-match-designated-marksman-rifle-dmr-trigger.html
Fully adjustable down into the upper ranges of the Match trigger, and up into the lower ranges of the Service trigger. It rocks.