What caliber? Easy, .30 caliber. Remember claiber doesn't = cartridge. As for which cartridge which I'm guessing is the question that was intended, I'd say .30-06 or .308 just for ease of ammo availability. Myself, I prefer the less common chamberings and would opt for something along the lines of a .257 Roberts, .405 Winchesteror .300 H&H.