Headshots did work for me. double tap to kill
But the upgrading eq didnt seem logical nor did I have ANY idea what it did. New gun. still double tap., though it might have had more bullets...
More story and less zombiekills would be recomendable. Maybe a "total amount of zomibes still alive" like 10.000 -> 9.900 after first mission then maybe 9.830 if you fail next level?