Cool game. I think the jump could be tweaked a little. I liked the sprites. Also a nice choice of music.
Interesting choice to have the gun push you back when firing. Overall, I think this turned out pretty well.
But I did encounter a bug in the station stage. If you get to the floppy disk in the station stage and turn around and go back to the previous room, you will be stuck in the door as it has closed again.