Four of the five greatest massacres in US history did not involve "Assault" weapons. One of them just barely involved a weapon at all.
Two of them relied entirely on explosives made from commercially available materials. One was committed entirely by handguns. One was committed almost entirely with a hunting rifle. The one that did involve an "assault" weapon would have still occured without it (assuming the weapon had been unavailable) as the perpetrator had two handguns with him that would have done the same thing.
An "assault" weapons ban would have mitigated exactly nothing in any of these massacres.