Per Bod:
Tell me how many mass shootings that an armed civilian has prevented or
have shot the perpetrator?
All the mass shootings I've heard about in the US, the perpetrators have
been shot by the police.

I would think that nobody here knows anything like that off the top of
their head - so whoever tries to determine it will be Googling.... and,
logically, that would be the one who wants to know.

--
Pete Cresswell