Just a Couple of suggestions
First : switch the motion controls to AWSD because it sort of difficult to move and reload when i've already got one hand on the mouse shooting. It would allow for easy reloads if i could just tap it quickly with my thumb while the AWSD keys are at my finger tips.
Second : Make it longer it's a cool style of game, I liked the ranking system you had at the end with the grade. It would be cool to see what sort of average you could get over several lvls
Third : The music...it got reather annoying really quickly, check out the audio portal for something that suits the game well. Also if you do add more lvls then you should have different music as well, and switch it up to more intense music when you get to a boss.
With those couple of modifications this game could be come even better!