Each gunner moves around the perimeter of the web page and fires bullets towards your mouse pointer. That's it. There's no game to it. It's just a demonstration. But that doesn't mean you can't have a little fun with it! If you leave your mouse cursor perfectly still, every bullet will hit it.
Be careful. It's very easy to overload your browser with too many gunners.
You are more than welcome to take a look at gunner.js (12 KiB), download it, and use it on your own website.