Very neat game! I had fun beating it, but still wondering what that secret medal could be!
One point of feedback: I didn't really get what advantage the holding down to chargeup had, since it seemed like you could kill enemies faster by shooting quickly. One suggestion would be that the charged up bullets could set enemies on fire, causing passive damage over time. Then you could group enemies up, set them all on fire, and pick them off one by one.
Anyways, great job with the game!