One of the 'defining' elements of an assault weapon is the ability to add a 'grenade launcher'.
Google up "M203 40mm Grenade Launcher" and see how many you can find for sale to the civilian market. I have an AR15 that I use for High Power Competition shooting. I'm not alone. The AR15 is the most widely used rifle in not only High Power, but most rifle shooting competitions in the country.
While my AR could house the M203 Launcher I can't go to any store, gun show or private dealer in the country and get one. They are exclusively for sale to the military and para-military forces. If I'm buying one I'm dealing with a criminal. To add that in to the discussion on what should be prohibited is absurd. It already is.