I'm surprised they don't make a device that makes the sound of racking a shotgun. If that doesn't work use it as a club.
I don't know much about rubber bullets but the bean bag projectiles used in shotguns sounds like a good choice.
Add: Now that I think of it, any "non lethal" weapon can likely result in death if the circumstances are right. Do you really want the scum around to sue your boots off after the fact?
Link to happy sack:
http://www.keepshooting.com/ammunition/speciality/bean-bag-round-shotgun.htm