Well that was a fun experience :D Killing all enemies with one bullet is really satisfying. The menu design is also great.
The only thing that bugs me is why we have to pick the weapon everytime it just doesnt add anything to the game at all? It just adds extra steps which is annoying :/ Also maybe adding a laser pointer or preview of the shot can make the game more responsive.
Other than these i enjoyed my time with this one you should port this to mobile nice work :D