Background <p>Research on the effectiveness of comprehensive background check requirements for firearm purchases often finds individual-level associations that do not translate to the population level. However, few studies assess the effects of these policies on nonfatal firearm violence.</p> Methods <p>First, several imputation strategies were compared to mitigate the effects of systematic missingness of crime data reported to the Federal Bureau of Investigation by law enforcement agencies. Next, the imputed data and the augmented synthetic control method, adapted for multiple treated units, were used to estimate the effects of implementing comprehensive background check policies on nonfatal firearm violence in five states. The study period began 15 years prior to the enactment date for each state and extended through 2020.</p> Results <p>The results were largely null across multiple specifications and sensitivity analyses.</p> Conclusions <p>Background checks are central to many firearm policies in the United States, yet several studies, including this one, do not detect an association between comprehensive background check policies and state-level firearm violence rates. These findings highlight the need to understand how the design, implementation, and enforcement of these laws can be improved.</p>