Assault weapon platform lethality in US mass public shootings 1966–2025

Abstract Firearm violence is a leading cause of premature death worldwide, and mass public shootings sit at the far tail of severity. Research has linked higher casualty counts to assault weapons, but the category enters those studies as a single yes/no indicator, so the weapon characteristics behind the association remain unidentified. We analyzed 197 US mass public shootings from 1966 through 2025 (202 perpetrators, 433 firearms, 288 fired) recorded in The Violence Project Mass Shooter Database, splitting the assault weapon category into weapon platforms, cartridge energy classes, and magazine capacity. Assault weapon use held between 17 and 20% of incidents per decade from the 1970s through the 2000s, then rose to 36% in the 2010s and 60% in the incomplete 2020s, with the shift concentrated after 2015. In adjusted negative binomial models, assault weapon use was associated with 55% more fatalities and roughly four times as many injuries; excluding the 2017 Las Vegas shooting cut the injury estimate to 2.3. The AR-15 was the only platform associated with both outcomes, but an additive model of platform and cartridge is rejected in favor of an interaction (likelihood ratio P < .05 for both outcomes, with and without the Las Vegas outlier), and neither component carries the association alone. Excluding Las Vegas and adjusting for magazine capacity, incidents in which an AR-15 fired a rifle-class cartridge had 1.6 times the fatalities and 2.4 times the injuries of the reference category, while rifle-class cartridges fired from other platforms showed none. Large-capacity magazines were independently associated with both outcomes, at 1.4 for fatalities and 1.8 for injuries, and the magazine estimate for fatalities moved little across every model we fit. What the binary classification records as one exposure is a compound of platform, ammunition, and magazine capacity, and the components do not behave alike.