I had a lot of fun with this one.
Although the graphics are (intentionally) extremely low-res, the color palettes are excellent, and the overall result is a world that actually looks rather nice. The music is some rollicking honky-tonk madness that I enjoyed the whole way through.
The boss design was hilarious.
One small recommendation: More feedback when the player's bullet strikes the boss. A flash of color, perhaps? A louder sound?