Why doesn't riot police use machine-guns with rat-shot rubber ammo?

I think it's a brilliant idea to use several machine-guns that fire rat-shot rounds with rubber balls.Mount the machine-guns on APC or whatever vehicles you have for riot policing and you've got yourself a very efficient system.Another way to do it would be to use AA-12s with beanbag ammo or rubber ball ammo or soft slugs.Why don't they use it?I mean rat-shot rounds,especially rubber ones are so small and soft,that even if they would penetrate skin they would still go no deeper than like half an inch or something.