Do Citizens With Guns Ever Stop Mass Shootings?

Short Answer: Yes
Longer answer: One can rarely be certain about such things...but almost certainly.

   When I first saw the title of this, I thought "that's the wrong question." I thought that because (1) genuine mass shootings of the relevant kind (as opposed to, say, certain kinds of gang-related violence) are extremely rare. And (2) concealed-carry permits are extremely rare. So the intersection of (1) and (2) are very rare indeed...
   But it turns out that there is still enough data to make an affirmative answer possible.
   Every time something like this happens, I think that more reasonable people ought to carry firearms...  But carrying a gun is a massive pain in the ass...and, of course, about half the population would think you were a lunatic if they ever found out that you were doing it...no matter how good your intentions... Not, I suppose, that that should matter...
   I hate dipping my toe into this discussion, because both sides are utterly, completely, totally insane when it comes to guns.
   So I'm just going to shut up now.