Great Job.
The varying levels of difficulty, the gun types, the ranks for scores, and the music selection all made this a surprisingly nice game for a barebones shooter. I do have to admit that the constant headshot thing was kind of annoying, but overall, good job. Keep it up. If you make a sequel, I would recommend different locations, multiple kinds of enemies, more guns, and maybe a new gameplay feature or two.
I'm submitting it to a collection, by the way.