The United States is ranked first in gun possession globally and is among the countries suffering the most from firearm violence. Several aspects of the US firearm ecosystem have been detailed over the years, mostly focusing on nation- or state-level phenomena. Systematic, high-resolution studies that compare US cities are largely lacking, leaving several questions open. For example, how does firearm violence vary with the population size of a US city? Are guns more prevalent and accessible in larger cities? In search of answers to these questions, we apply urban scaling theory, which has been instrumental in understanding the present and future of urbanization for the past 15 years. We collate a dataset about firearm violence, accessibility and ownership in 929 cities, ranging from 10,000 to 20,000,000 people. We discover superlinear scaling of firearm violence (measured through the incidence of firearm homicides and armed robberies) and sublinear scaling of both firearm ownership (inferred from the percentage of suicides that are committed with firearm) and firearm accessibility (measured as the prevalence of federal firearm-selling licenses). To investigate the mechanism underlying the US firearm ecosystem, we establish a novel information-theoretic methodology that infers associations from the variance of urban features about scaling laws. We unveil influence of violence and firearm accessibility on firearm ownership, which we model through a Cobb–Douglas function. Such an influence suggests that self-protection could be a critical driver of firearm ownership in US cities, whose extent is moderated by access to firearms.