I appreciate the difficulty of only moving with the recoil of the weapon. One thing that I didn't really like was being killed by enemies that appear suddenly without me having a chance to avoid them - sometimes without even seeing them at all. I'm not sure what the best way to avoid this would be though. Maybe ensuring that the enemies always spawn a certain distance from the player, or not adding the enemy collision box until the enemy has been on screen for a second or two.
The pixel art is awesome btw.