Fun little game, though it could use a little polish. The main thing is that gears block your shots, so if the lead zombie in a swarm drops one, all the zombies behind him are shielded. You can fix this by putting pickups and drops in separate groups (select the mob/pickup scene's root node, click the node tab in the inspector, then switch to Groups), and setting the bullet collision to only trigger if it collides with something in the mob group (i.e. if collision_body.is_in_group("mob"): trigger_collision). Other things I noticed is that the shield doesn't seem to work either, and you can keep buying heals when you're already at full HP. Keep up the good work!