Here's the fly in the ointment about stopping a mass shooter.
They aren't a mass shooter until they begin shooting. So until then, they haven't actually done anything to justify deadly force to stop them. Brandishing a firearm and/or actually threatening people can be justification but until/unless anyone who is armed and in a position to intervene recognizes the the threat, they don't know to do anything.
And that's the rub, once they start shooting, everyone recognizes the threat, so then it becomes a matter of time (and location) and the capability of the armed citizen that matters. But generally speaking, the shooting has to start before that happens and that means that the shooting was NOT prevented. It can mean the shooting was stopped after it began so fewer people get shot. but that's not "preventing" the shooting and so that doesn't count in a database about preventing such shootings.
After the shooter finished in the church and left, he was confronted, and that prevented him from doing any MORE, but not from doing what he had already done before armed response arrived.
Same to a degree in the other shooting, the shooting wasn't prevented but it was stopped before it got any worse. He was able to shoot a couple people before being stopped, so the shooting wasn't prevented, but it was STOPPED.
and I think my point about the "massive wave of first time gun buyers" is a valid one. Some of them will, hopefully take up the cause of gun rights, because they are now gun owners. However there are a lot of people in this country with the attitude AB mentioned. "I got mine, sucks to be you!"
I wouldn't count on any of them to support gun rights.