Layering is broken. If there is an enemy behind another one, the first one will die, then I will keep shooting its corpse instead of the guy behind him until the corpse disappears. Fix pl0x.
Also, when I lose a level, the music "slows down". All I get from my headphones is a series of pops. My headphones are good quality, too, so I am certain it has to do with the sounds you are trying to output. Too low frequency or just too loud.
Gets a 2/5 for now. Switching ammo is damn near impossible. After my first ammo drop, I tried it out, didn't like it, so wanted to go back to standard. You have to STOP SHOOTING to switch ammo, because it cycles through types you don't even have. Get rid of that. Holding Space is also a bad idea. Just have it hit on/off.