This was pretty fun! Having the gun change regularly make the game quite spicy, and the different kind of enemies are interesting to fight. The last level is especially interesting, and satisfying to beat.
Perhaps the game is "too balanced" though? Most of the powers are efficient, but not overkilled. So it never changes the way to play. In the end, I just shoot bullets at enemies. If they do greater damage, or are more precise, whatever, it just means it'll go quicker. I wish I would feel more the change, and that each new weapons would force me to think about how to use them.
But it was still a fun shooter in the end, with a nice twist and a nice visual and sound atmosphere. Good job!