Statement of Purpose Firearms ended the lives of almost 40,000 people in the United States in 2017, including over 14,000 by homicide. Stronger state firearm laws are associated with fewer firearm homicides and are an essential element of a comprehensive approach to preventing firearm homicide. However, flow of firearm across state lines can undermine prevention efforts in states with strong firearm laws. The purpose of this study was to assess the flow of firearms across US states relative to state firearm laws.

Methods The Bureau of Alcohol, Tobacco, Firearms and Explosives provided data for all firearms that were used in crimes for 2015–2017 and could be traced to a retail purchase location. We created a 48x48 matrix of the contiguous US states for counts of firearm transfers for origin states (where firearms were first purchased) and destination states (where crimes were committed). We linked these data to 2015 firearm laws for origin and destination states, and assessed associations between laws and transfers using Exponential Random Graph Models applied to this valued directed geographic network.

Results A total of 178,712 interstate firearms were transferred between the 48 contiguous US states for 2015 to 2017. After accounting for network structure and population characteristics, additional firearm laws in origin states were associated with 12% fewer traced firearm transfers (IRR=0.88; 95%CI: 0.83, 0.93, p<0.001), whereas additional firearm laws in destination states was associated with 10% more traced firearm transfers (IRR=1.10; 95%CI: 1.06, 1.15, p<0.001).

Conclusion Weak firearm laws affect firearm availability in states with strong firearm laws due to illegal flow across state lines.

Significance/contribution to Injury and Violence Prevention Science Firearm homicide disproportionately affects minority and otherwise disadvantaged individuals, groups, and neighborhoods. Preventive intervention to reduce health disparities related to firearm homicide must consider human mobility.