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It has been hypothesized that the harms of mass shootings in the US could be mitigated or prevented by encouraging gun ownership and reducing legal obstacles to gun carrying.

With approximately 400,000,000 (that's 400 million) guns in the hands of American citizens, and on average, more than 1 gun owned per person (including children), what percentage of mass shootings in the USA have actually been stopped by ordinary citizens?

The USA uses various definitions of mass shootings, but for this question, we can use the common definition of mass murder (4 or more people killed) or the broader Stanford Mass Shootings of America (MSA) definition (3 or more people injured).

I'm interested in data from any time period during the last century spanning at least 1 year.

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    I think it is more complicate than simply put gun numbers without check the facts - how many actual gun owners with at least one fire arm, how many gun owners have carrying permit, and how many people carry gun with them 24/7. Finally, where were the mass killing occurred vs the local gun laws. Note I am not a NRA supporter, nor I support non-conditional gun ownership, just feel this is a much bigger/complex issue than that can be viewed with a conclusion be drawn from the statistics alone. . – r13 Mar 23 at 13:30
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    @r13 You did hit on the problem with this question. I know more people who don't own guns than do and the ones I know that do own guns own a lot of them. From what I know about the people who do own guns it isn't hard to make a guess that not as many people own guns as the numbers suggest as many gun owners own multiple guns. Trying to average the guns owned per person gives a really inaccurate picture of gun ownership. – Joe W Mar 23 at 13:46
  • I think this question needs a linkage that's currently missing (like the one from @Jontia 's comment). If no one is claiming that owning a yellow car (for example) stops mass shootings, then there's no point in a question about whether yellow cars stop mass shootings. (There wouldn't be a point to a question asking whether yellow cars cause 20-car pileups either unless someone was making that claim, even though they're both car-related.) – Bobson Mar 23 at 15:56
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    Lots of comments deleted. This is not a place to debate gun control. Please keep your comments relevant to the purposes comments should be used for. – Philipp Mar 23 at 23:41
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    Related question: How often do private-owned guns protect US-Americans. – Erwan Mar 24 at 23:47
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From the details in the The Violence Project database. The following can be seen.

  • In six incidents, armed civilians were present at the location. (Armed Police were present in 19 Incidents) out of 175 recorded incidents.
  • The average number of people who were killed and injured in these 175 incidents are 7.18 dead and 11.6 injured.
  • For shootings with an armed civilian on site, the average are 6.9 dead and 8.3 injured. These numbers are influenced by one extreme example with 15 dead. If we remove that, then the average is 5.2 dead and 3.8 injured.

Statistics based on the available sample say that the ordinary gun-bearing citizen's presence in the location has an insignificant effect on the outcome. The same has been highlighted by many.

This data is based on incidents which resulted in 4+ deaths. The statistics of incidents which did not result in death might show a completely different result. I could not find any such data so far. However, from the available data the effectiveness of an armed civilian in preventing mass shooting can be inferred to be insignificant.

Update FBI Article Active Shooter Events from 2000 to 2012 provides the details of "active shooter" Incidents which by definition is slightly different from what is considered above. The summary on how these incidents ended is worth considering for the analysis.

Manner in Which Incident ended

The point to highlight is that in 3 cases the shooter was shot dead by the Armed Citizen and in 14 cases he was overpowered. The data provides scope for interpretation. The sample size is fortunately low and we wish that it stays low. The overall answer is that the hard statistics available in the public domain are inconclusive. However based on available data it can be said 3 to 4 percent of incidents have been concluded by armed ordinary American.

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    @Jontia That doesn't seem to answer the question at all. What I am asking about is did the presence of an armed civilian actually stop the shooting? What I see in the answer just talks about someone being present and not them actually having an impact on stopping it. It doesn't also include if the shooter could have caused harm by either getting themselves shot or shooting someone else which could increase the number of victims. You can make stats say pretty much anything so just saying that the stats say it doesn't help – Joe W Mar 23 at 21:09
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    Another issue is that this doesn't consider shooting events where the perpetrator was stopped immediately. If a shooter is stopped before he hits 4 or more people, it won't be considered a mass shooting. A kind of selection bias it seems. – Ryan_L Mar 23 at 21:44
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    I was going to write my own, but since you've found the FBI reports and I'm lazy: a few more FBI reports, covering 160 incidents in 2000–13, 40 in 2014–15 and 50 in 2016–17 (IDK if they've published a 18–19). Of the first 160, 5 were ended by armed non-law-enforcement, 4 of which were security guards. 1/2 – timuzhti Mar 24 at 6:51
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    Of the next 40, 2 were ended by non-security-personel and one armed civilian pursuing was shot and killed before he could fire. Of the final 50, 4 armed civilians were successful (two of them exchanged fire, two held the shooter at gunpoint), one exchanged fire causing the shooter to flee to another scene to continue shooting and one was wounded before he could fire. 2/2 – timuzhti Mar 24 at 6:54
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    If you feel that the example with 15 dead skews the results, it might be helpful to list median numbers as well as mean. – Kyralessa Apr 14 at 8:38
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Unfortunately, statistics based on actual events are not able to answer this question.

A working hypothesis is that a higher rate of gun ownership among targets will discourage potential shooters from trying. Similarly, easier access to guns will encourage potential shooters to try. The policy can clearly be counter-productive if only partially implemented, i.e. low rate of gun ownership in the target community, easy access to guns in the potential shooter's community - which seems a difficult challenge to address in settings where the vast majority targets cannot be armed, e.g. in a Kindergarten (some lawmakers disagree).

Creating meaningful statistics based on past events to support or reject this hypothesis is infeasible, because any such comparison (e.g. shootings in Dallas-Fort-Worth vs shootings in New York) will be subject to a massive number of factors that the study can't control, as well as a low sample size, which makes such a study useless, except for scoring political points.

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  • Comments are not for extended discussion; this conversation has been moved to chat. – JJJ Mar 25 at 0:30
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There are several factors that make this a very difficult question to answer definitively.

  • What exactly is a "mass shooting"?
  • What exactly does it mean to stop a mass shooting?
  • How to account for mass shootings that are prevented by armed citizens, or even by the possibility that armed citizens will be present?
  • Should other similar violent crimes be accounted for as well in considering policy?

Here is just one example of how jockeying over definitions can affect reporting and public perception. Depending how these questions are answered, which reasonable people might disagree on, an intervention by an armed citizen to detain or kill a would-be mass shooter, that prevents or minimizes other casualties, might not be counted in statistics as a mass shooting at all.

In which of these hypothetical scenarios would an armed citizen have "stopped a mass shooting"?

  • A would-be shooter enters a public place that allows citizen carry and opens fire, killing five people before an armed citizen returns fire killing the shooter. Six dead, including the shooter, others wounded. Similar real-life example from headline news.
  • A would-be shooter enters a public place that allows citizen carry and opens fire, killing one person and wounding several others before an armed citizen returns fire killing the shooter. Two dead, including the shooter, others wounded. According to the above linked article, the FBI would not consider this to be a mass shooting, as less than four people were killed.
  • A would-be shooter enters a public place that allows citizen carry and opens fire, wounding five people before an armed citizen returns fire. The shooter flees the scene and is later arrested. None dead, several wounded. Similar real-life example.
  • A would-be shooter enters a public place that allows citizen carry and opens fire, wounding five people before an armed citizen returns fire killing the shooter. One dead, the shooter, others wounded. Similar real-life example.
  • A would-be shooter enters a public place that allows citizen carry and opens fire, inflicting no injuries. An armed citizen returns fire, striking the shooter. The shooter then drops his weapon and is detained by the citizen until police arrive to arrest him. None dead, one injured, the shooter. Similar real-life example.
  • A would-be shooter enters a public place that allows citizen carry and opens fire, inflicting no injuries. An armed citizen returns fire but misses. The shooter drops his weapon and is detained by the citizen until police arrive and arrest him. No casualties. Real-life example, reported as "foiling" a mass shooting.
  • A would-be shooter enters a public place, draws his firearm, and is immediately shot and killed by a well-trained armed citizen before he can open fire. This would only be counted as a mass shooting event if the definition included the potential for harm, and possibly not even then. Real-life example.
  • A would-be shooter enters a public place, draws his firearm and begins shouting loudly. An armed citizen draws his weapon and orders the suspect to drop his. The suspect complies and is detained until police arrive to arrest him. No shots fired. This would only be counted as a mass shooting event if the definition included the potential for harm, and possibly not even then.
  • A would-be shooter enters a public place, with the intention to open fire, but upon seeing armed citizens openly carrying firearms, instead simply leaves without incident. No shots fired. Such an occurrence would probably never be recognized as a potential mass shooting event, much less reported and counted.
  • A would-be shooter enters a public place, with the intention to open fire, but upon seeing a sign prominently displayed, "We support the Second Amendment / Firearms Welcome", instead simply leaves without incident. No shots fired. Such an occurrence would be purely the result of policy, not even of citizen action, and probably would never be recognized as a potential mass shooting event, much less reported and counted.
  • A would-be shooter enters a public place that allows citizen carry and begins stabbing people with a large knife, killing five people before an armed citizen opens fire killing the attacker. Six dead, others wounded. Real-life example. This would probably not be counted as a mass shooting event, but should such a scenario be considered for purposes of policy?
  • There are many more occurrences of defensive use of firearms in response to home invasions. These are not generally thought of as mass shootings, not being public places, but the nature of the crimes involved is similar, and the intent of gun control laws is to reduce the number / types of guns and the amount of ammunition in citizens' hands, whether they are in public or at home.

Many advocates of citizen carry believe that encouraging citizen carry is an effective deterrent to those who might be otherwise inclined to attack the vulnerable. It is difficult to directly measure and prove this effect; it seemingly can be measured only by comparing broad crime statistics between jurisdictions with similar demographics but different citizen carry policies.

To the extent this effect is real, it is without doubt to be preferred as a means of stopping mass shootings over direct intervention by returning fire after a perpetrator opens fire. But arguments over definitions and the difficulty of knowing how many times potential shootings were prevented by citizen carry, or simply by allowing citizen carry, make it difficult to have an objective debate in the public square.

Ultimately, it comes down to politics, how you believe our society should be. Do you trust your fellow citizens to carry firearms, or not? Do you trust your government and police to protect you from the bad guys, or not? Is individual liberty so important as to make these other questions irrelevant, or not?

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  • Comments are not for extended discussion; this conversation has been moved to chat. – JJJ Mar 27 at 0:11
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There are two major problems with attempting to answer this question:

  • the absence of a uniform consensus of what constitutes a mass shooting
  • the lack of a consistent primary source of data spanning all incidents – attempted or otherwise partially / fully committed

That is to say, databases that track mass shootings record and publish data in uniquified ways, and thus there's not even a consensus on the number of mass shootings that occurred within a given timeframe. For instance, the most authoritative compilers of mass shootings – the Gun Violence Archive, Mother Jones, the FBI, and the Stanford MSA – have databases of "mass shooting" that wildly deviate from one another. For 2017, the total number of "mass shooting" victims enumerated by these sources ranged from 612 to 2200+!

enter image description here

Even when factoring and filtering the databases to unify onto a single strictest definition of "mass shooter" incidents results in discrepancies:

enter image description here

Data compiled into these libraries also come from various sources, including but not limited to: media stories, research data sets, the scientific literature, books, advocacy groups, press data sets, police records, FBI records, CIA records, court documents, the BJS, and accounts detailed by the offenders. There is no centralized, dependable, consonant record of every incident that led to 3+ or 4+ deaths that occurred in any year nationwide, let alone an authoritative entity that has record of every attempted mass shooting that could have potentially caused 3+ deaths.

enter image description here

My point is that any dataset that you view is likely to be flawed or incomplete, although the post above with the FBI's sequence diagrams is likely the closest you'll get to an answer.

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