The point being that when you avail the purchase or ownership of semiautomatics to the general public, you are essentially making the devastation greater for a potential massacre event.
I don't think this is correct at all. First of all if I was gong to do a mass shooting... I would use a shot gun not a semiautomatic rifle/handgun. Also the worst mass murders of all time had nothing to do with guns. Stuff like fertilizer, box cutters, or even gasoline and a match have proven to be MUCH more effective than firearms.