I only just realized that those bullseye symbols on the enemies are where you need to hit to get a kill, and that you end up with more points if you fire ONE bullet into the head or chest.
Wow, awesome game. Just about the only improvement would be replacing Artillery with something that isn't a less effective and heavier version of Single Rockets :P
Protip: GET A LASER. The first one availible can 1hit anything in World 1, can fire through anything, and is only listed with low damage because it's weak. For a laser.