Temporal trends in public mass shootings: high-capacity magazines significantly increase fatality counts, and are becoming more prevalent

Background: There has been disagreement in the literature regarding whether public mass shootings are becoming more frequent, or more deadly, and whether high-capacity magazine (HCM) weaponry is associated with increased fatalities. Objective: We examined the 85 non-felony- and non-gang-related mass shootings with at least four people shot and killed that occurred in public places between 1995 to 2018 to examine the frequency and deadliness of public mass shootings, as well as the role of HCM weaponry in fatalities. Findings: The per-capita rate of public mass shootings with at least four people killed has not significantly increased since 1995 (Negative Binomial linear regression p=0.20), but public mass shootings with at least six people killed have been increasing by 6.9% per year (p=0.01). Public mass shootings are becoming significantly more deadly, with fatalities per incident increasing by 4.5% per year (truncated Negative Binomial log-linear regression p<0.001). Use of HCM weaponry banned during the 1994 to 2004 Federal Assault Weapons Ban (FAWB) increased fatality counts on average by 1.9 times, and total number shot by 3.3 times (non-parametric bootstrap comparison of means (NPBCM), p < 0.001 in both cases). In these incidents, HCM pistols were as deadly as assault rifles. The fraction of firearms involving HCMs used by perpetrators more than doubled after the lapse of the federal ban, from 25% during the ban to 59% after (Binomial factor regression, p<0.001). Our online visual analytics application, https://mass-shooting-analytics.shinyapps.io/mass_shootings/, allows interested parties to examine and download our data, replicate our analysis results and explore the analysis with alternate selections. Policy recommendations: A re-instatement of the federal ban on high-capacity magazines in pistols and rifles, or special taxes on such weaponry, may reduce public mass shooting fatalities. Keywords: mass shootings; assault weapons; high-capacity magazines; temporal trends; Federal Assault Weapons Ban