Great idea, although i didn't really like that my hp was going down so fast. Maybe i was missing something, but i was just reloading for hp bullets. It would've been great if my hp wouldn't shrink while i did not already shot the red and green squares.
So i might have skill issue, but i couldn't really figure out how to play the game as intended. I was mostly just waiting for enemies to spawn and reloading for heal, i couldn't ever live for too long. If you educate me a bit, i could give it another try.