.30-06 all the way. about the most available ammo for large rifles out there and can do anything you need. african game specialists used them to control a hippo population some years back with tremendous effect. WOULD i use it to take down big bears or african game....not a chance! CAN it do it? of course...it has. but none of these are coyote to moose...their days are numbered against a hunter wielding a .30-06.
while the surplus ammo out there leans to the .308 win, there is PLENTY of surplus -06. you can also find it anywhere. also, if you lose your weapon when the zombies hit and have 2000 rounds of ammo, what are you more likely to find lying around in a farmhouse you take shelter in? gotta be an old -06.