Well if it's truly for long range, the best size bullet for ballistic coefficient, which equates to best downrange retained energy and least amount of drop would be a 130 grain bullet in a 270. (.277 inch diameter). Find a load tailored around that bullet weigh. Every gun will shoot every bullet/combination different, so while getting ideas here is great, remember that Joes gun will spit the same load out much differently than yours.
Federal makes some premium ammo with Barnes X bullets, Trophy bonded bear claws and a few others. I have had good luck with them. Heres the page.
http://glarp.atk.com/2007/2007_Catalogs/FederalPremiumCatalog.pdf