Mercy Hospital shooting: bad guy with a gun stopped by good guy with same
Story here. The killer murdered a psychiatric case worker, and then was shot and seriously wounded by a doctor who was carrying a gun. The doctor was nicked by a shot from the killer, it's not yet reported whether that came before or after he hit the killer.
This may illustrate what others have noted: the other side can claim that mass shootings have not been stopped by self-defenders, because mass killings are arbitrarily defined to involve four or more dead, and when a self-defender is present he or she stops the attack before it can reach that number.
UPDATE: the doctor got three hits on the murderer. I'd say the good guy was no amateur.