honestly for zombies i think .22 LR would be your best bet overall. its cheap, easy to find, lightweight (so you can carry TONS) and does a good job at penetrating the skull.

its quiet compared to just about everything else, so you wont alert other zombies.

get one of those lakeside machine .22lr machine guns to mow em down.. lol

smaller calibers would probably be the best way to go...unless you are looking at shooting distance, where the larger calibers excel.

when youre fighting off huge waves though, something closer to either a shotgun or p90 might be best.