An analysis of every US mass shooting over the past 30 years shows that two small policy changes, restricting high-capacity magazines and introducing stringent background checks, might have saved over 500 injuries and deaths, reducing total casualties in mass shootings by 50%.

Various proposals have been set forth since 2012’s numerous mass shootings, ranging from much stricter gun regulations to arming more individuals in public spaces. Starting from Mother Jones’ list of US mass shootings over the past 30 years, I analyzed the potential impact of two proposals in particular: would a ban on high-capacity gun magazines have reduced casualties, and would stringent background checks of gun purchasers have reduced the number of shootings? I researched the incidents surrounding each shooting to determine whether each proposal would have had any impact. The data are summarized in the table below, with the full research spreadsheet available here.

Shooting	Deaths	Injuries	Lives Saved	Injuries Prevented	Weapon Legally Acquired?	Notes
Totals:	459	481	250	324	Legal in 58 of 63 cases	54% of deaths and 67% of injuries might have been prevented with the policies analyzed.
Newtown, CT Sandy Hook Elementary	27	2	17	0	Yes – legal weapons in same household	The shooter’s rampage was stopped by a quick police response. If the shooter had to reload 3 times as often, he would hit many less victims, as he fired on each victim multiple times.
Minneapolis, MN Sign Company	6	2	1	0	Yes	The shooter reloaded at least once during the shooting, and initially struggled with victims.
Oak Creek, WI Sikh Temple	6	4	3	2	Yes	In a public setting with many adults, it’s possible shooter would have been stopped while attempting to reload, or would have retreated outside more quickly if he had less capacity.
Aurora, CO Theater	12	59	12	59	Yes	A lucky form of weapon capacity control prevented a larger disaster, as the shooter’s weapon jammed and he was only able to fire roughly 1/3 of the 100 round magazine. A properly integrated background check system might have stopped the incident entirely.
Seattle, WA Cafe	5	1	0	0	Yes	It’s not clear that the gunman ever needed to reload, and though he had a history of mental health problems, he was never treated and never convicted of a felony.
Oakland, CA – Oikos University	7	3	7	3	Yes	HCM limit would have no impact here, but the shooter was expelled from school for behavioral issues, which might have been caught if this data were submitted to a comprehensive background check system.
Atlanta, GA – Health Spa	4	0	4	0	Yes	HCM limit and background check would have no impact here
Seal Beach, CA – Salon	8	1	3	0	Yes	The shooter reloaded during the shooting per police reports, so lowering weapon capacity would likely have lowered casualties.
Carson City, NV – IHOP	4	7	2	3	Yes	The shooter fired over 30 rounds per eyewitness accounts – lower capacity would have constrained him.
Tucson, AZ – Giffords shooting	6	13	4	9	Yes	Shooter was tackled and stopped while he tried to reload – direct evidence that lower capacity would have decreased the toll.
Manchester, CT – Beer Company	8	2	4	1	Yes	Shooter used two weapons and fired multiple rounds at many victims – had he been limited, he would have run out of ammunition earlier
Lakewood, WA – police officer shooting	4	0	0	0	No	Capacity limits might not have helped, as the shooter fired on four victims seated at one table, and hit all of them with his initial salvo.
Ford Hood, TX – army base	13	29	9	19	Yes	Shooter reloaded many times, and 30 round magazines enabled him to fire roughly 170 rounds before being shot himself by military police. Multiple soldiers attempted to charge the shooter – if he had only a 10 round magazine, it’s entirely possible that he would have been tackled and stopped upon initial reload.
Binghamton, NY – civic association	13	4	9	3	Yes	Shooter fired 99 rounds in total – this would likelybeen reduced if his weapon capacity were 1/3 as large
Carthage, NC – nursing home	8	3	0	0	Yes	Since shooter used multiple weapons and never reloaded, it’s unlikely capacity limits would have mattered.
Henderson, KY – Atlantis Plastics	5	1	0	0	Yes	Shooter did not use a high capacity weapon
Dekalb, IL – Northern Illinois University	5	17	5	17	Yes	This incident’s casualty count is quite low because the shooter first fired with a very low capacity weapon, his 6-round shotgun – enabling many students to escape the classroom. Shooter also had a long, documented mental health history.
Kirkwood, MO – City Council	6	1	0	0	Yes	Shooter used low-capacity revolver initially, and took a higher capacity weapon from a victim (police officer).
Omaha, NE – Westroads Mall	8	4	5	3	No	Shooter appears to have emptied one magazine and then taken his own life.
Crandon, WI – sheriff’s rampage	6	1	0	0	Yes	Shooter used a service weapon, so proposed rules/limitations would have had no effect.
Blacksburg, VA – Va. Tech	32	23	32	23	Yes	Shooter reloaded many times, and used multiple weapons. Mental health check would have prevented weapons acquisition.
Salt Lake City, UT – Trolley Square	5	4	5	4	Yes	Shooter did not use high capacity weapons
Nickel Mines, PA – Amish School	5	5	2	2	Yes	Once shooter started firing, sheriffs approached – he killed himself as they arrived, and likely would not have had a chance to reload.
Seattle, WA – Capitol Hill	6	2	6	2	Yes	Shooter had a weapons-related felony charge, which was reduced to a misdemeanor.
Goleta, CA – postal shooting	6	0	6	0	Yes	Shooter had a previous history of mental illness
Red Lake, MN – high school	9	5	3	2	Yes	Shooter possessed a gun in his bedroom despite being treated with Prozac. Since he was an adolescent, and his parents/guardians chose to give him a gun, background checks would be ineffective. Shooter shot his grandfather who was a police officer, and took his weapons.
Brookfield, WI – Church group	7	4	2	1	Yes	Shooter suffered depression, but had no mental health or criminal records.
Columbus, OH – concert	4	7	0	0	Yes	No HCM used, and no medical or criminal record. Nearby police stormed the concert and shot suspect
Meridian, MS – Lockheed Martin	8	7	4	3	Yes	Shooter used military-style weapon with high-capacity
Melrose Park, IL – Navistar	4	4	4	4	Yes	Shooter used military-style weapon with high-capacity, and was also a convicted felon
Wakefield, MA	7	0	5	0	Yes	Shooter used high-capacity weapon and also had a history of mental illness, but with the mental illness far in his past and no criminal record, even stringent checks might not have denied him weapons. Shooter stopped firing at an arbitrary point and sat calmly til arrested. If he had lower capacity weapons, stopping to reload multiple times might have caused him to sit and wait for arrest earlier.
Tampa, FL – hotel	5	3	5	3	Yes	Shooter was arrested for assault only a few months earlier, and bought weapon at a gun dealer
Honululu, HI – Xerox	7	0	3	0	Yes	Shooter acquired a large number of weapons long before mental issues began.
Fort Worth, TX – Wedgwood Baptist Church	7	7	2	2	Yes	Shooter committed suicide after emptying three magazines – but he had six more loaded. Has the magazines been 1/3 smaller, that would have lowered the toll proportionally.
Atlanta, GA – Day trading	9	13	0	0	Yes	The shootings happened in multiple separate incidents, making it less likely that HCM limits would have had an impact. Barton was suspected but never charged in earlier murders, so background checks would have had no impact.
Littleton, CO – Columbine High	13	21	6	10	No	Shooters used a high capacity Tec-9 and standard capacity 9mm, so avg capacity is used here. Details of the shooting indicate that in many cases shooters fired at the same victim multiple times – if limited in capacity, this would have reduced their ability to fire on additional victims.
Springfield, OR – Thurston High	2	24	1	19	Yes	Shooter was tackled and stopped when he first tried to reload – a clear indication that lower capacity would have further limited casualties.
Jonesboro, AR – Westside Middle School	5	10	2	3	Yes	Shooters ran away after firing 30 rounds – lower capacity might have reduced total rounds fired.
Newington, CT – Lottery worker	4	0	0	0	Yes	Shooter chose specific victims and fired relatively few rounds, so capacity limits make no difference here.
Orange, CA – Caltrans	4	2	3	1	Yes	Shooter entered shootout with police shortly after initial incident, lower capacity might have shortened his attack
Aiken, SC – RE Phelon Co	4	3	0	0	No	Standard capacity weapon (illegally acquired) used
Fort Lauderdale, FL – city employee	5	1	0	0	Yes	Standard capacity weapon used
Corpus Christi, TX – Walter Rossler Co	5	0	0	0	Yes	Standard capacity weapon used
Fairchild AFB, WA – hospital	5	22	5	22	Yes	Shooter possessed only one 75 round drum magazine – so he would never have to reload. Military police arrived quickly and killed perpetrator.
Aurora, CO – Chuck E Cheese	5	0	0	0	No	Shooter fired less than 10 times, executing each victim, usually with a single shot
Garden City, NY – LIRR	6	19	2	6	Yes	Shooter emptied two 15 round magazines and was tackled while reloading with a third magazine. Total rounds fired would have been decreased by 1/3 were magazine capacity limits in place.
Fayetteville, NC – Luigi’s Restaurant	4	6	2	3	Yes	Shooter used a high capacity rifle, shooting was stopped by nearby police
San Francisco, CA – 101 California St office building	8	6	4	3	Yes	Shooter used a 32 round Tec-9 in the shooting, and fired hundreds of rounds
Watkins Glen, NY – office	4	0	0	0	Yes	Shooter killed four intentional targets with relatively few shots, and then waited for police to arrive – perhaps less than 10 shots total fired.
Olivehurst, CA – Lindhurst High School	4	10	0	0	Yes	Shooter used two weapons and fired relatively few shots, so high capacity weapon limits would have no effect here. Shooter also had no prior criminal or mental history.
Royal Oak, MI – postal	4	6	4	6	Yes	Shooter had his concealed weapons permit revoked on concern of mental illness. Shooter also used high-capacity magazines with his rifle and fired scores of rounds according to police.
Iowa City, IA – Univ of Iowa	5	1	0	0	Yes	Did not use a high-capacity weapon, and did not display sufficient signs of mental illness prior to shooting to warrant attention
Killeen, TX – Luby’s Cafeteria	20	24	8	10	Yes	Used high capacity pistols and reloaded multiple times – capacity limits would have enabled more victims to escape, as many escaped by exiting the restaurant.
Jacksonville, FL – GMAC plant	9	4	9	4	Yes	Shooter had a history of violence and convictions, and yet legally purchased multiple weapons. Used a high capacity weapon in shooting
Louisville, KY – Standard Gravure Co	8	12	8	12	Yes	Shooter used high capacity weapon, emptying its magazine and committing suicide with his second weapon. Shooter also had a lengthy psychiatric history including hospitalization
Stockton, CA – schoolyard	5	29	5	29	Yes	Shooter had a lengthy arrest history and had served time in jail as an accomplice to armed robbery, and yet was allowed to buy weapons.
Sunnyvale, CA – ESL Co shooting	7	4	7	4	Yes	Shooter was able to purchase guns while under a court restraining order
Palm Bay, FL – shopping center	6	14	6	14	Yes	Shooter used a high capacity .223 caliber rifle, and killed two police officers during the shooting – one of them as the officer was trying to reload. Perhaps if the shooter’s capacity were lower, the officer might have himself fared better. Gunman also had prior assault conviction.
Edmond, OK – USPS	14	6	0	0	Yes	Shooter was in National Guard and would have had access to weapons. Though he was referred to as “Crazy Pat”, he had no history of crime or treated mental illness
San Ysidro, CA – McDonalds	21	19	14	13	Yes	Shooter used a high capacity weapon, Uzi, pinning down a quick-responding officer with 30 rounds of fire before re-entering restaurant
Dallas, TX – nightclub	6	1	0	0	Yes	Shooter used an unknown handgun, emptying it into crowd and then rushing out – unclear that capacity limit would have any impact here.
Miami, FL – welding shop	8	3	8	3	Yes	Shooter did not use a high capacity weapon, but purchased his weapons one day after failing a psychiatric exam ordered by his employer, the school district, and after incidents in which he appeared to be a threat to students
Birchwood, WI – hunting altercation	6	2	3	1	Yes	Shooter fired 20 rounds at other hunters – if he had a lower capacity, it’s likely that another hunter would have been able to respond with fire

The analysis above attempts to answer the question – what would have happened in these incidents had the proposed laws been in place? Of 459 deaths and 481 injuries in 63 shootings, I estimate that 250 deaths and 324 injuries (54% of deaths and 67% of injuries) might have been prevented with the analyzed proposals. Each proposal, its method of action, and the analysis approach is described further below.

High-Capacity Magazine Ban:

Definition: Sales of high-capacity magazines to and between private citizens would be completely banned, and imports of high-capacity magazines for private use would be banned as well. While many magazines would exist in private hands, a magazine buyback could then be used effectively, as magazines are relatively inexpensive.

Method of Action:

1. In some instances, the shooter was disarmed by potential victims while trying to reload – smaller magazine size clearly would have limited total impact in these shootings.
2. In some instances, potential victims fled during breaks in the shooting enabled by reloading – if a shooter has to reload 2 or 3 times as often, this effect is multiplied.
3. In some instances, law enforcement arrived relatively quickly, and most damage in the shooting was done via the initial magazine – a smaller magazine would have limited impact in the shooting in these instances.
4. In a few instances, victims attempted to rush the shooter immediately. If a shooter could only fire 10 shots instead of 20-50, it’s possible that he might be tackled quickly rather than be able to continue shooting.
5. In most instances, the shooter committed suicide after doing a certain amount of shooting, but always before exhausting ammunition. Since each reloading represents a break in the act, some shooters would commit suicide after having fired fewer total rounds if they were capacity constrained.
6. In a few instances, the shooter appeared to choose a specific weapon because of its high capacity. If high capacity magazines were not available, would the shooter still go forward with the attack?
7. In 18 of 63 shootings, shooters fired relatively few rounds, chose a small number of specific victims, or used standard capacity weapons. In these instances the high-capacity magazine ban has no impact. 29% of actual mass shootings fell into this category.

Analysis Method: If the shooting fell into the last category above, then zero impact is noted in the analysis. Otherwise, the casualty count is reduced by the ratio of the shooter’s magazine size to standard magazine size – if the shooter used a 30 round magazine, then the casualty count is estimated at 2/3ds lower (rounded up) with a standard capacity magazine. This approach will tend to underestimate the effect of a ban in instances like 1,4, and 6 above, while providing an accurate estimate or an overestimate in instances like 2, 3, and 5 above. In aggregate, I think this approach is unbiased.

Stringent Background Checks:

Definition: Create a mandatory national database of all felons, mentally ill, and others posing threats (anti-terror lists, those who have made threats against schools or other institutions). Mandate that all firearms transactions for new and used weapons, in public and private transactions, be checked against this database, with instant results. This stands in contrast to the current background check system, which is done on paper and via telephone call, not electronically.

Method of Action:

1. Out of 63 mass shootings over the past 30 years, only 5 have involved illegally purchased weapons. Some of the shooters had a history of mental illness or a criminal record – preventing a sale of firearms to these individuals would reduce the frequency of shootings.
2. Many of the shooters with a history of mental illness had no criminal record – it’s unlikely that they would know how to obtain an illegal firearm.
3. Some of the shooters purchased weapons in the days after making threats against a school or other institution – in these cases, a properly implemented stringent background check system would have prevented the weapon sales.

Analysis Method: Shootings were identified in which a shooter had a documented history of mental illness, a criminal record, or had made threats against an institution prior to buying a weapon. In these cases (17 instances total) it’s assumed that the casualty count is reduced to 0, as the shooter would have been unable to obtain a weapon. In reality a certain number of shooters would then try to acquire weapons illegally, and some might succeed. But a certain number of mentally-ill or former felons might never try to obtain a weapon if they knew they had no easy or legal means to do so, providing an offset.

Analysis of Assault Weapons Ban and Armed Civilian Presence

Two other proposals have been mentioned in the last several months – a ban on assault weapons and the placement of more armed guards or civilians in public places. On the question of assault weapons, the data from mass shootings shows that shooters preferred a range of semi-automatic weapons with high-capacity magazines. Weapon capacity makes a difference, but the type of weapon (handgun vs rifle) does not.

With regard to armed bystanders, in 9 of 63 shootings armed individuals (often police officers) were present. In several cases armed individuals became victims in the shooting, and the presence of armed individuals did not prevent the shooting from taking place. However, this analysis is by definition incomplete – this is an analysis of shootings that actually did take place, and doesn’t include data on shootings that were stopped by armed individuals. The evidence here suggests that the element of surprise may render concealed weapons somewhat ineffective, but this is not a conclusive finding.