[In] 33 percent (17) of the cases that ended before the police arrived, the potential victims at the scene stopped the shooter themselves. Most commonly they physically subdued the attacker (14 cases), but 3 cases involved people at the scene shooting the perpetrator to end the attack.

So it appears that even the FBI says there were 3 over a span of 12 years that were foiled by armed citizens. And that fully one third of them were stopped by citizens without guns.

I would assume that if everybody were armed then, that more than 33 percent would be foiled by armed citizens.

That is of course, if you accept the Federal Bureau of Investigation to be a credible source

http://freedomoutpost.com/2015/10/since-2009-92-percent-of-m...