Great Start! I truly do love what you have here, but it's not really playable.
-The main issue is a game breaking bug that will not let you progress. Just one part of the screen will just not respond to shooting. The first part I noticed this happens is the third group of enemies that appears when you select the swamp under the bridge in the Forest level. The "zombie" on the right cannot be shot, so you just die.
-Reload needs to work anywhere at anytime. If this was intentional as a mechanic to stop "Shoot/loading" then I get it, but running out of ammo right when you need it is a frustrating killer.
-Middle mouse click to go into cover? That just doesn't fit well here. Middle Mouse is always going to be fickle when it wants to work. Even if you just add a keyboard button or something that would be much appreciated.
-Also, when you die, you cannot return to the main menu, it just seems like you click it and it does nothing. Restarting seems to work fine.
Those are the main issues that need to be fixed the others are not as important, like animations for when the "zombies" come out of the ground, projectiles being much closer than they should be when thrown. Things like are good polish, but not necessary.
I really do enjoy this game and I mean this to be encouraging, you have made a great thing here and I'm sure many players would love to see your work in a better light.
Have a great day and thanks for making cool games!!!