.450 Bushmaster, .458 SOCOM and .50 Beowolf are all fine cartridges for 200 yards and less and you'll stay in the AR-15 platform. Anything more than that and you'll want to up it to an AR-10 which can be chambered for calibers from .243 Win to .338 LM. A good .308 WIN AR-10 that you can hunt with will run you more than a bolt gun, but can be configured to other uses.