In a semi auto I would look at the various AR-10's available.

DPMS has a good selection of .308 Ar's thatshould easily do 600 yards if you do your part.
http://dpmsinc.com/firearms/firearm.aspx?id=14

There are also many other manufacturers that offer very accurate AR platforms chambered in the .308 and a variety of other cartridges capable of 600 yard shots.