The game need some more variety.
You need to make it feel good to play. Maybe add some different guns, different cowboys.
Different power-ups. There are so many things you can add, the cowboys can have different way of dying.
The games needs fixing up, do that, and it will be good.

Your game has no 'omph'. Add sound effects, give it a lot of polish, and a reason to keep playing. I couldn't tell if there was more than one level and became bored instantly. It was also very hard to tell if I was doing good or bad, so give player more feedback on his performance. What you have here is a prototype, not a game.