While the statistical relationship between gun ownership and mass homicides has been discussed on Skeptics.SE, I have not seen any questions or answers on the following claim.

The policy of the National Rifle Association includes statements such as:

"The only thing that stops a bad guy with a gun is a good guy with a gun,"
— Wayne LaPierre, NRA's executive vice president.

and, specifically the following hypothetical,

"What if when Adam Lanza started shooting his way into Sandy Hook Elementary School" he had been confronted by a trained, armed security guard, asks [LaPierre]? He calls for all schools to have at least one police officer on site.

Arguably the presence of someone with a gun at a prospective target would act as a deterrent, but the claim seems to be that use of the guns by a "good guy" prevents mass killings, which (perhaps incorrectly) I take to mean that the "good guy" with a gun used it to kill a potential mass murderer before the killing began.

While the use of firearms has certainly stopped a number of mass murderers from continuing a killing spree that had already begun, are there any examples or records of statistics on the number of mass murderers having been prevented because there a "good guy used a gun"?

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    guy with gun sees other guy shooting, guy shoots his own gun=> it turns into a firefight with many innocent bystanders... – ratchet freak Feb 19 '13 at 17:18
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    I was going to answer, but this is really a duplicate.skeptics.stackexchange.com/q/14123/6876 also breitbart.com/Big-Government/2012/12/15/… is a very recent example. – Ryathal Feb 19 '13 at 18:17
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    It's ridiculous to say that once it has begun it doesn't count. A person who shoots someone because they think a mass shooting is going to occur is going to get in a lot of trouble. Preemptive strikes like that are very unlikely and arguably immoral when done by fallible beings. I think the scope should be expanded to include in progress shootings ended prematurely. – William Grobman Feb 19 '13 at 18:56
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    Of course, an interesting thing to note is that part of the answer is "no matter what the asnwer is, it will be "too few compared to how many could have been stopped", since most people intending mass murder choose to do it in "gun-free" zones that don't have ANY good guys with guns, by design. – user5341 Feb 19 '13 at 19:28
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    It's strange that more of them don't choose to do it in gun free zones like UK or Australia. – DJClayworth Feb 20 '13 at 14:23

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