If damage is not what you're looking for and you care more about dealing with as many zombies as possible in the most efficient way, then you probably need a weapon that has a very high fire rate.
Enter the Keris V10. This submachine gun is exactly that kind of weapon. It's not terribly accurate, especially in automatic mode, but if you're fighting against a couple of dozen zombies, then accuracy is the least of your worries.
With a 90 fire-rate rating, the highest on this list, the Keris V-10 can mow down the undead in scores.