IF he could not have acquired a rapid fire assault weapon, he might have killed 'only' 3-4 with his handgun...

I think it goes a step beyond that:

Most mass shooters do not delude themselves that they are going to walk away. They either accept the fact that they're going to be killed by the police or plan on killing themselves at the end of their sprees. So, they must ask themselves, "If I'm going to die, how many people do I need to take with me for it to be an 'even trade'." I daresay that, for most shooters, the figure is in the range that can only be achieved by using high-capacity semi-automatic weapons. So, removing such weapons would not merely have the effect of reducing casualties during shooting incidents, it would eliminate many such incidents from occuring at all. A deranged person who is willing to trade his own life for the lives of thirty people may well not be willing to make such a trade for the lives of three or four.