Ok it's not a joke and it's not a riddle but it is a puzzle (kinda). I will admit it's the opposite of funny.
http://artscool.cfa.cmu.edu/~lee/deanimator.html
2 tips.
1. It only takes 1 bullet if the Zombie is still emerging.
2. I managed to get 130 by resorting to rapid fire when they are bunched up.
Edit: just discovered you can change to a shotgun by pressing shift. Warning though, they get halfway across in the time it takes to reload the shotgun.