Prior Army sniper and instructor here. The 308 is the workhorse you are looking for. A good 308 setup will just about shoot itself out to 500 with decent ammo and glass. There is so much 308 data available for both factory ammo or roll-your-own options. Besides magnum calibers, 308 is the gold standard, and available in every imaginable configuration. From match grade rounds, to hunting rounds, to surplus military and military clone ammo, the options are endless. Optics are where you will (or should) spend the most $.