The 160
https://www.fbi.gov/file-repository/active-shooter-study-2000-2013-1.pdf
J. Pete Blair and Katherine W. Schweit, "A Study of Active Shooter Incidents, 2000 - 2013", Texas State University and Federal Bureau of Investigation, U.S. Department of Justice, Washington D.C., 2014.
This is a study of of what FBI termed
active shooter incidents -- individual or individuals actively engaged in killing or attempting to kill people in a populated area confined or unconfined. It is not a study of mass killings or mass shooting only 64 of the 160 qualified as "mass killing" (FBI definition 3 or more dead).
158 active shooters were "lone wolves" only 2 had partners
6 active shooters were women
37 active shooters committed suicide before police arrived
17 active shooters committed suicide after police arrived but before police could act
9 active shooters committed suicide after exchanging gunfire with police
only 6 active shooters wore or were carrying body armor
only 3 active shooters carried improvised explosive devices
"Of the 160 incidents, at least 107 (66.9%) ended before police arrived and could engage the shooter, either because a citizen intervened, the shooter fled, or the shooter committed suicide or was killed by someone at the scene."
21 incidents were ended after unarmed citizens restrained the shooter
5 incidents were ended after armed non-LE citizens exchanged gunfire with the shooter
"The individuals involved in these shootings included a citizen with a valid firearms permit and armed security guards at a church, an airline counter, a federally managed museum, and a school board meeting"
2 incidents were ended when off-duty police exchanged gunfire with the shooter
This 160 study by Advanced Law Enforcement Rapid Response Training Center at Texas State University (TXST) is geared for training police going into an active shooter situation. I am not sure that stats based on a narrow definition of "active shooter" speaks to whether private arms are a deterrent value in active shooter, mass shooter or terrorist attacks.*
The Sullivan County high school incident would not count. The in-school resource officer SRO held off at gun point a gunman who had entered the school. (The gunman had demanded the SRO's gun and wanted the school fire alarms sounded, so could have been a mass shooting situation.) As per training, other Sullivan Co deputies were at the school immediately when alerted and went in to engage the gunman. When ordered to disarm, the gunman swerved his gun toward the deputies then back to the SRO (herself a sheriff's deputy); he was promptly shot.
The resource officer at Columbine was outside the school and took his training as meaning call for SWAT and hostage negotiators rather than engage an active shooter.
I saved a documentary on the 21 Sep 2013 Westgate Shopping Mall Massacre, Nairobi, Kenya.
"Terror at the Mall" HBO CNN.
About 2000 people in the mall. CCTV footage, accounts of eye witnesses, make it clear these are tough events. Just like Columbine, the SWAT took hours of containing the perimeter and waiting for orders from headquarters before going in. Off-duty police and ex-military some with private arms went in on their own recognisance and rescued hundreds trapped in the mall (the police were in civilan clothes and their issue long arm was AK). The military finally went in and shot two policemen then withdrew after about 90 minutes.
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* Wright & Rossi could interview prisoners and ask if they knew of crimes deterred by armed citizens.
In the James D. Wright & Peter Rossi armed inmate survey (sample size of 1865 inmates, 18 prisons, 10 states)
81% will try to determine if a potential victim is armed.
57% said that they had encountered potential victims who were armed.
40% said that they had been deterred from a crime because they believed the victim was armed.
34% said that they had been scared off, shot at, wounded, or captured by an armed citizen.
74% indicated that burglars avoided occupied dwellings because of fear of being shot.
57% said that most criminals feared armed victims more than they feared police.
I do not believe there has been an attempt to ask active shooters if private arms were a deterrent or changed their plans. The sample size of surviving active shooters is too small to be statistically significant any way.
For terrorists, they do publish lists of preferred targets. The Jihadist publication Rumiyah, ninth issue:
"Ideal target locations for hostage-taking scenarios include night clubs, movie theaters, busy shopping malls and large stores, popular restaurants, concert halls, university campuses, public swimming pools, indoor ice skating rinks, and generally any busy enclosed area, as such an environment allows for one to take control of the situation by rounding up the kuffar [non-Muslims] present inside and allows one to massacre them while using the building as a natural defense against any responding force attempting to enter and bring the operation to a quick halt."
They don't list gun shows or gun clubs or shooting ranges for some reason.