Are most mass shootings done with illegal guns?

1. The definitional problem that shapes every headline

Any attempt to measure whether mass shooters use “illegal” guns starts with a fraught definitional landscape: there is no single, universally agreed definition of “mass shooting,” and datasets vary in inclusion rules, which changes the denominator of incidents under study ^{[4] [5] [6]}. Reporting organizations from Gun Violence Archive to The Violence Project and academic teams use different thresholds (three or four people shot, public vs. private incidents, exclusion of gang or domestic contexts), and that heterogeneity alters both the apparent frequency of weapon types and the documented provenance of weapons ^{[7] [5] [3]}.

2. What kinds of guns are actually used in mass attacks

Multiple empirical sources find that handguns are the most common weapon type in mass shootings, with rifles accounting for a much smaller share of firearm murders overall; the FBI and public databases report handguns comprising the plurality or majority of guns recovered in mass incidents ^{[4] [2] [1]}. Studies of publicly targeted fatal mass shootings likewise found handguns were most frequently present and that many events involved multiple firearm types rather than a single “assault weapon” ^[8].

3. Legal versus illegal: what the datasets record (and what they don’t)

Some research resources and compiled databases record whether weapons were purchased legally, illegally, or stolen, but coverage is uneven and often relies on open-source reporting or law-enforcement documents rather than a single federal register ^{[3] [9]}. The Violence Project and NIJ-supported datasets include fields for whether weapons were obtained legally or stolen, while surveys and policy reviews referenced by RAND point to DOJ inmate-survey work that attempts to track source and use—but these efforts do not uniformly capture every incident or clarify whether a legally purchased gun became “illegal” due to a later prohibition on the owner ^{[3] [10] [9]}.

4. What the evidence says about the prevalence of illegal guns in mass shootings

Across the public literature there is not a clear finding that a majority of mass shootings are committed with weapons that were illegal at the time of the attack; rather, many well-documented mass-shooting firearms were obtained through regular retail channels, private sales, or theft, and the balance varies by incident and dataset ^{[3] [9] [8]}. Scholarly work has highlighted cases where “assault weapons” added lethality, but even those studies note that many mass events use handguns and that presence of a particular firearm type does not map neatly onto legal status ^{[8] [1]}.

5. Alternative interpretations and policy implications

Advocates focused on gun trafficking and crime emphasize illegal markets and straw purchases as key sources of firearms used in violent crime, while public-health researchers stress safe storage, access, and the role of legally purchased guns in many incidents; these perspectives are not mutually exclusive because datasets show mixed sourcing patterns and significant data gaps ^{[10] [11]}. Without comprehensive, standardized federal data on the legal status of each firearm at the moment of every mass shooting, policy debates are forced to rely on partial evidence and differing priorities among sources such as Gun Violence Archive, academic studies, and advocacy groups ^{[7] [12] [3]}.

6. Bottom line — direct answer

No: the best available, publicly reported data do not show that most mass shootings are done with illegal guns; instead, mass shootings most often involve handguns and a mix of legally purchased, stolen, or otherwise obtained firearms, and the record lacks the comprehensive, uniform sourcing data needed to make a definitive national percentage about “illegal” guns in mass shootings ^{[1] [2] [3]}. The evidence base supports targeted reforms (safe-storage laws, background checks, enforcement against trafficking) but cannot, as of the sources available, sustain a simple, categorical claim that most mass shootings are committed with illegal firearms ^{[10] [3]}.