Okay, yeah .22 is great if all you get is headshots. But if we're talking about the crazy fast ferocious zombie type good luck placing ALL your shots in the head. I've been thinking about it and I'm gonna go with a .30-06 with a fat bolt so you can "tommy finger" it.

Also, .30-06 is pretty common. Not as common as 22LR but you probably won't have any trouble keeping stocked, provided you survive.