There are beanbag rounds and I think you (if LEO) can get rubber buckshot, buckshot and CS gas, but all of these are in my opinion not used by most who mount these on AR's.
If you need a less leathal launcher, it may be better to get a 37mm stand-alone system with it's own stock so you don't have to point a loaded AR at a person you don't intend to kill.
37mm is basically a compliance tool for law enforcement, the 40mm can also me offensive.
In the AR mounted role I think the 40mm launcher is more versatile, as they can take, HEDP, buckshot, flechette, CS, smoke, illumination, and (I think) even riot/less lethal ammo.
If you want it for looks just buy a high quality airsoft M203 with Colt marked reciever, it will give you the "right look" (which for anyone who has handled a 203 is not something you will get with a 37mm)