Don't head shots work best?
Anything upwards of 20mm doesn't need targeting that proper.
As the fine people at cracked.com found out (
LINK),...
However, since you're in a tank I suppose encountering burning zombies is not too much of a problem.
I'd also vote for canister. :)
Or, if you're crazy enough and the zombie crowd is big enough, how about ye olde chain-shot?
What was good against pirates can't be bad against zombies!
Hmmmm, silver canister against werewolves?