April 2012, Aurora CO Church shooting stopped by off-duty cop in congregation
http://www.huffingtonpost.com/2012/0...n_1446969.html
I am not sure how you came up with this one being a potential mass shooting since the article title specifically says the congregation probably wasn't the target.
April 2012, Salt Lake City mass stabbing:stopped by armed citizen:
http://www.abc4.com/content/about_4/...iNMfH.facebook
A stabbing isn't a mass shooting.
January 2002, Appalachian School of Law shooting stopped by armed citizens:
http://en.wikipedia.org/wiki/Appalac...f_Law_shooting
The shooting event was over by the time the two students (both off duty cops taking law courses) retrieved firearms from vehicles and found Peter O already tackled and downed by a a real life non-cop student who did not have a gun.
August 2012, TX mass shooting stopped by armed citizen (including saving the life of a cop who was pinned and taking fire from a gunman):
http://www.guns.com/texas-gun-owner-...out-10236.html
Maybe, maybe not. He killed two people and a couple dogs (which don't count). Stacy purportedly saved one officer as per the sheriff, but the officer had been fired at multiple times and not hit and had fired multiple times at Conner and apparently not hit Conner. So it was a standoff at that point in terms of exchanging shots.
There have been mass shootings or attempts at places that were known to be armed:
1. Tacoma Mall - a shall issue state. Good guy screwed up.
2. Tyler Courthouse - LEOs a plenty.
3. Hunting - the VietNamese BG who took on hunters that he felt insulted him.
4. Google on armed robberies at gun stores - they do happen.
5. Rampages where folks enter police stations - google it.
6. Gabby Gifford's - a shall issue state and CCW types present.
7. Texas Tower - in those days, guns aplenty on campus or nearby.
8. Colorado church - shall issue locale and a success for the good guys.
There were no police and no CCW people at the Giffords event. It took the first officers some 4 minutes to arrive. Joe Zamudio was not at the event, but managed to show up with his gun undrawn, claimed to have nearly shot the guy holding the gun taking from Loughner, saw the shooter was already downed by people present who didn't use guns, ran over and held Loughner's feet and tried to call 911 and could not get through. In short, he made a big deal about having a gun and being the hero, but arrived late to the party and while he did help, a lot of what he claimed isn't confirmed. By his own admission, Loughner was down when he arrived, however.
The guy killed the people with whom he had a conflict and was going to fight the cops. It seems a stretch to call such incidents potential mass shootings without saying that any gunbattle with the cops is a potential mass shooting because they virtually never have just one responding officer.
The same with Tacoma Mall. McKown didn't even have he gun drawn when he yelled at the gunman, was shot multiple times, but takes credit for stopping the guy because after shooting McKown repeatedly, the guy retired to a record store and took hostages before surrendering. How McKown takes credit for stopping the guy is beyond me.