This project’s main purpose was to facilitate research that will further understanding on firearm legislation and its impacts on violence. The researchers completed a comprehensive data collection effort that included information on firearm legislation implemented across U.S. States over time in combination with multiple measures of firearm-related violence and injury. To better understand the conditions under which firearm legislation is more or less effective the researchers incorporated information on State and county-level indicators of firearm prevalence, firearm shops, enforcement of firearm laws, social norms about firearm ownership, demographics on racial and economic inequality, and access to healthcare, emergency care, and mental health services. Incorporation of these county characteristics allowed for examination of whether the effectiveness of State-level firearm legislation depends upon particular characteristics of counties. These data also allowed for leveraging for change in both firearm legislation and county characteristics over time to better understand how firearm legislation emerges and impacts firearm violence. Without these data, policy makers have little ability to propose evidence-based policies that maximize safety and reduce morbidity/mortality without infringing on constitutional rights.