Do Citizens With Guns Ever Stop Mass Shootings?

Yes
   It's really the wrong question--it's asked in an overly-restrictive way. But still the answer ends up being in the affirmative.
   Many on the anti-firearm left like to assert that, in such cases, the citizen responding with lethal force is (a) guaranteed to not be able to stop the shooter, and (b) guaranteed to massacre the other innocent potential victims...and, quite possibly, every other human person on the face of the planet...    Because that's how firearms work...they can only harm innocents...
   But, in case any reasonable person out there is for some reason tempted by that fantasy tale...as Volokh, it's false.
 
   I shouldn't be so snarky about this. IMO derision is very effective at entrenching disconfirmed belief. We all tend to find it irksome to be proven wrong...and it's even worse if our interlocutor dances about on the grave of our cherished belief...  I certainly don't think that the anti-firearm left is wrong about everything. I think they're righter than the gun-lovin' right about many things. But the Better Off Unarmed argument is just. so. damned. stupid... It's really difficult to avoid derision in the face of such godawful thinking.
   [Insert standard complaining about counterproductivity arguments here]
   The anti-firearm left would do better to drop this absurd argument. All they're doing is making themselves look dumb. They've got much stronger points they could make. IMO they should emphasize the small number of CCW-holders, the fact that many of them are poorly-trained, etc. But pretending that it's actually worse if one of the potential victims is armed is just delusional. (Of course there's a more sensible version of the argument according to which the presence of an armed citizen merely fails to improve the situation... But that argument is also crap.)