The issue really isn't the gun though. If he would have went in there with 4 of these
http://en.wikipedia.org/wiki/Benelli_M4 each with a 2 round extender tube on it and firing #00 buckshot I am quite sure the bodycount would be significantly higher. Throw in a few handguns for when he fires all 28 shotgun shells and the entire situation doesn't really change.
The fact is this guy was fucking crazy and hell bent on killing people. That wouldn't have changed, nor would the situation ended "better", had he not been able to buy an evil black rifle. Hell, if he was firing slugs from the shotgun you are almost guaranteed 1 dead for every hit.