From a practical (cost/benefit) standpoint, I like a .30 caliber rifle. My own rifle is an old WWII era .303 Enfield that's been 'sporterized' with a modern stock and a scope. A 30-06 or a .308 work well, and I think an SKS would also be a good option if you're budget minded. Of course, as others have pointed out, if you hit them in the brain with just about anything they're pretty much yours. I would think a .222 would be okay, the key is, whatever you're shooting, to use soft point ammo. FMJ just doesn't do as good a job and I'd consider it inhumane. Your odds of wounding and not finding are greater with the FMJ ammo.