Conservatives (and pro-Second-Amendment liberals, and yes, they do exist) frequently say "a good guy with gun beats a bad guy with a gun". 

The left poo-poohs that saying. 

But new evidence shows the conservatives are right; civilians with firearms are disproportionately effective at stopping spree killings.   John Petrolino at our sister publication Bearing Arms:

Dr. John R. Lott, Jr. from the Crime Prevention Research Center and Dr. Carlisle E. Moody, Professor of Economics, Emeritus from College of William and Mary, aggregated the data. Their paper, “Do Armed Civilians Stop Active Shooters More Effectively Than Uniformed Police?” was released on April 3.

Not only are civilians effective; according to the paper, they are at least incrementally more effective than law enforcement:

“The first takeaway is, assuming our count is complete, that armed citizens have stopped more active shooter incidents than the police have, although the difference is not significantly different from zero,” Lott and Moody noted. “Also, armed citizens do not appear to interfere with the police or blunder so badly as to get their weapon taken away by the shooter or kill the wrong person.”

Moreover, one of the more significant findings was that “according to police, armed citizens have stopped 57 active shooter events which otherwise were likely to have escalated into mass public shootings – where ‘many’ people risked being murdered.”

Petrolino and the paper both stress that this is not a ding on law enforcement; they are almost never there when the incidents begin.  That's by design; the shooters plan their sprees that way, and in some cases specifically take out the police or armed security who are on the scene at the beginning of their rampages; as the study points out:

[Police are] less likely to be near an attack when it occurs, and when they are, they’re more likely to be targeted and killed...The concern over these tactical disadvantages is the reason that air marshals on airplanes do not wear uniforms. Ideally, we would also compare non-uniformed officers to officers, but sample of non-uniformed officers is too small, with only a couple cases.

Even when the police are on the scene almost immediately and take the correct action, as they did in the Antioch High School shooting in Nashville two years ago, it gives the killers minutes to wreak their havoc.

Beyond that? Law enforcement have known since the Columbine shooting that the best way to tackle a spree killing is to go straight in after the shooter; like the cops in Nashville, to grab a rifle or shotgun and, covering each other, move to engage.  Spree killers, who frequently spend months or years planning their rampages, are frequently in a dissociated reverie state as they make their attacks; resistance of any kind frequently causes the reverie to break, and even if the shooter isn't killed, they often break off their attacks, and give up or commit suicide.  

But does it work?

That question brings us back to the study. 

The list of armed, non-law-enforcement citizens who have ended likely mass / spree killings is a lot longer than you'd expect, if your source is the mainstream media.  The subject violates the narrative; armed citizens aren't supposed to be effective.  At the very best, they are supposed to be bumbling incompetents who are a greater danger to innocent bystanders or cops than to the killers - a view the study also flenses:

Gun control advocates raise the concern that concealed handgun permit holders who stop an attack might accidentally shoot a bystander. They are also fearful that the police might accidentally kill the permit holder.  While such incidents are a possibility, they have yet to occur.  We probably do not have all the cases where a permit holder stopped a mass public shooting, but if a permit holder were to shoot a bystander, it seems clear that such an event would get news coverage.

So - underneath all of that, what do the data show that the Good Guy with a Gun accomplishes?

[The paper] shows that armed citizens reduce the number of deaths in active shooter incidents significantly more than the police do. In fact, armed citizens reduce the number of people killed by 49 percent while the police increase the number killed by 16 percent in comparison to the omitted class (shooters who are arrested later or stopped by unarmed citizens or stop of their own accord). This does not mean that calling the police results in more deaths. The police are associated events that are more deadly than either armed citizen response or other action. However, the results indicate that police are less effective than armed citizens in reducing the number of deaths associated with active shooter incident. 

This is just one of a number of observations; the paper is not the heaviest reading by John Lott standards, but it's not something to bring to the beach unless you're, like, me.  

But give it a read.  In between the lines, you can hear leftist narratives about self-defense screaming for mercy.