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I've heard it several times, "More than 80% of mass shootings are carried out with handguns", often on social media, for example:
Handguns are responsible for more than 80% of mass shootings
...and sometimes in passing mentions in articles, for example:
...handguns are responsible for more than 80% of total mass shootings
...but I haven't been able to find any corroborating evidence.
Furthermore, I've found evidence that contradicts the claim here, but maybe I'm missing something.
The main problem with statistics like these is that there is no one definition of "mass shooting" and thus, by adjusting your parameters, can make the number be whatever you want to fit a narrative.
What is a mass shooting? There are some events, like the Stoneman Douglas High School shooting, the 2017 Las Vegas shooting, and the Columbine High School massacre, where everyone can agree that it should be considered a mass shooting.
But what about this incident in Saginaw, Michigan, where 5 people were shot at a house party and no one was killed? Or this event in Madison, Alabama, where 4 people were shot and no one was killed?
The Wikipedia article on Mass Shooting points out the huge issue in the definition of Mass Shootings.
The term was originally defined as the murder of four or more people with no cooling-off period but redefined by Congress in 2013 as being murder of three or more people. According to CNN, a mass shooting is defined as having four or more fatalities, not including gang killings or slayings that involve the death of multiple family members. In "Behind the Bloodshed", a report by USAToday, a mass killing is defined as any incident in which four or more were killed and also includes family killings. A crowdsourced data site cited by CNN, MSNBC, The New York Times, The Washington Post, The Economist, the BBC, etc., Mass Shooting Tracker, defines a mass shooting as any incident in which four or more people are shot, whether injured or killed. As of November 2017, the FBI defines a mass shooting as an incident involving "four or more people shot at once." A noteworthy connection has been reported in the U.S. between mass shootings and domestic or family violence, with a current or former intimate partner or family member killed in 76 of 133 cases (57%), and a perpetrator having previously been charged with domestic violence in 21.
The lack of a single definition can lead to alarmism in the news media, with some reports conflating categories of crimes.
PolitiFact agrees that there's a huge issue with that definition. In a 4 October 2017 article titled "How is a ‘mass shooting’ defined?", PolitiFact refuses to rate a claim by House Minority Leader Nancy Pelosi where she claimed
"273 mass shootings in 2017 -- one for each day of the year."
PolitiFact, before getting into the meat of the article, includes this sentence.
We are not rating her claim on our Truth-O-Meter, however, because our current research indicates the debate over the definition of mass shootings is more unsettled than ever.
They continue along in the article as if it was another fact check, although without an actual rating.
Pelosi’s spokesman told us the congresswoman’s number comes from the Gun Violence Archive, a crowdsourcing website that tracks gun deaths using media reports. It defines a mass shooting as one in which at least four people are injured or killed in one location, not including the suspect.
The Gun Violence Archive listed 273 mass shootings as of Oct. 1, 2017, the day of the Las Vegas massacre.
(...)
Pelosi’s claim and the mass shooting definition it rests on ignores strict criteria developed in a July 2015 Congressional Research Service report. It defines a mass shooting as "a multiple homicide incident in which four or more victims are murdered with firearms, within one event, and in one or more locations in close proximity." The report has been frequently cited by some criminologists and gun rights advocates.
Criminologists who spoke with PolitiFact in the past say this definition was more in line with events such as the shootings at Columbine, Virginia Tech or in San Bernardino.
They also told PolitiFact that the Gun Violence Archive’s tally includes far more than what would be considered mass shootings because it lumps in gang shootings and home invasion robberies.
As PolitiFact Florida reported in 2015, "Politicians or others who want to make a point about guns choose a set of data and a definition that reinforces the point they want to make. People who want more gun control tend to choose more expansive definitions." (emphasis mine)
PolitiFact continues by explaining that many criminal experts have their concerns even with these definitions.
Gun control groups say it’s arbitrary to distinguish between a death and an injury. They point out a significant problem: Some shootings that injure a dozen or more people but don’t kill four people would not be considered a mass shooting under the more restrictive definition.
"I would submit that sometimes the only difference between a shooting and a murder could be a centimeter, an inch, an unlikely ricochet, whatever," Bueermann, who now is president of the Police Foundation, which researches law enforcement practices, told the Post. "If we're trying to capture true gun violence in our country, a broader definition [of mass shooting] is probably more useful than a narrow one."
(Frederic) Lemieux went on to say the broad definitions of mass shooting remain problematic, citing the inclusion of crimes such as familicide, where "victims are exclusively family members and not random bystanders."
"Gang murders are usually crime for profit or a punishment for rival gangs or a member of the gang who is an informer. Such homicides don’t belong in the analysis of mass shootings," he added.
Gun Violence Archives brings this into greater clarity with their statistics for 2018. They use the "4 or more injuries" criteria to define a mass shooting. By this count, they have 103 mass shootings as of 24 May 2018. However, by using the more strict definition of "four or more victims murdered with firearms, within one event, and in one or more locations in close proximity", the count is at most 10. However, of the 10 you have
With the exception of the two school shootings, the Waffle House shooting, and the Car Wash shooting, many of these wouldn't even be considered mass shootings under this more restrictive definition. Some only hit the 4 deaths mark due to including the shooter, some take place in multiple locations, some involve family members, and some are possibly gang-related.
In terms of the 80% statistic you mentioned, Louder With Crowder is misreading a New York Times article where they stated that 80% of total gun homicides were done with a pistol. While it is unclear what time period, exactly, they are talking about, they seem to be quoting early 1990s homicide statistics.
See the New York Times
Handguns were used in more than 80 percent of gun murders each year
Compared to what Crowder said
handguns are responsible for more than 80% of total mass shootings
He even links to that article while claiming the statistic, which either means a misreading or deliberate attempt to deceive.
The New York Times, for the most part, seems to be backed up by the Department of Justice, who keeps track of homicides by gun type.
Year Handgun Homicides
1994 82.7%
1995 81.9
1996 81.1
1997 78.8
1998 80.4
1999 78.8
2000 78.6
2001 77.9
2002 75.8
2003 80.3
2004 78.0
2005 75.1
2006 77.0
2007 73.6
2008 71.8
2009 71.3
2010 69.6
2011 72.9
In terms of the type of weapon used in Mass Shootings, to illustrate the differences in definitions, I found three different reputable sources online that each reach different values.
Statistic seems to present a number closer to 51%.
Mother Jones puts it at 49.6%.
CrimeResearch.org says that 76% of mass shooters have a handgun on them during the event, but count any event where the shooter is in possession of a handgun, regardless of whether or not it is used.
