The FN 303 is a semi-automatic less-lethal launcher manufactured by Fabrique Nationale de Herstal. (FN)
The FN 303 uses compressed air to fire projectiles from a 15-round drum magazine. It is designed to incapacitate the target through blunt trauma without causing critical injuries, and is most widely used for riot control and other kinds of combat where lethal weapons should be avoided. The 303 can be fired from the shoulder using adjustable iron sights, or it can be mounted in an under-barrel configuration on most assault rifles when its stock assembly is removed (in this configuration, it is designated M303).
To the right are some of the different types of rounds available. You can't quite tell from the image, but the gray part of the 303 round is made up of tiny balls of Bismuth, (lead-free lead) and the clear part shatters on impact, which diminishes the force. I haven't been shot with one, but I'd imagine it still doesn't feel very good. For better info on the 303 ammunition and much better images of the rounds, check out this link to FN's site.
The Davenport Police Department has several of these, and I believe that at least 2 cars have them out on the street at any given time. I am not entirely sure of this, and maybe someone more in the know could correct me. What I do know is that the intended audience/targets, IE scumbags, gangsters, etc, know all about this weapon. During the unfortunate tenure of QCLive in the Freight House, they had a weekly event which I forget the name of. It might as well have been called gang member night, so the DPD would usually send a small force down there around closing time. Sometimes an officer would simply carry around the FN303, and it would scare many people off, or keep them from trying anything. The sight of the launcher alone is usually enough to frighten people who haven't seen it in action before.
Hopefully we're not headed for a time where police need to all carry these weapons, but even then I guess we'd still be ahead of the many countries where police are armed with AKs or submachine guns.