I have this same argument with my partner at work regularly. He feels that since the old timers killed many elk with a .30-30, then there is no reason to go with a .300 mag or whatever you might choose. I feel that as hunters we owe it to the animal to kill it as quickly and humanely as possible, and this generally requires "using enough gun". I could easily kill an elk with a .243, but what if my shot placement isn't perfect? I'd rather have the comfort of a cartridge that offers better penetration and energy. I would have to imagine that the same goes for zebra or anything else.