Hi Peerless (& other forum members)
I looked up the Luby's shootings and found the following:
Luby's massacre - Wikipedia, the free encyclopedia
I'm not sure if the conclusion of this story proves your point, about possible massacre prevention, but it does show that gun ownership should only be allowed following a psych evaluation.
UK Bob
PS. I understand that semi-automatic weapons can be very easily converted to fully automatic by the purchase of gun kits, which I understand are legal.