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"?