I like the idea and its impressive what you were able to do in this short time!
In my person experience with shooters, I think your bullets are not fast enough so when the target are moving you have to aim quite a bit in front of them. On top of that you can not see where the bullet goes, so you have no indication on where the shot went and in which way to improve.
At the same time I do like the difficulty of having to aim ahead though, it makes you feel like you are sniping a moving target from a great distance!
I also encountered a bug in the third level. I couldn't click on try again, it would just play the "game over" sound.
But great work! Keep it up!