a ar is what i was thinking dpms has a model that is set up for the military snipersthe Panther™ LRT-SASSit cost is 2100 though stock but it comes with everything you need minus the scope.
I was looking at bushmaster but there really only set at a few cals so not sure i will go with them even though i love there ars
ORIGINAL: bigbulls
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.