Philip J. Cook & Jens Ludwig, Guns in America, 1996, available at http://www.policefoundation.org/pdf/GunsinAmerica.pdf. 2,568 individuals, including 251 gun owners, were surveyed by telephone and asked how they obtained their firearms.
 In the 38 states that have not fully closed the private sale loophole, an estimated 9,856,984 background checks were conducted between November 2011 and November 2012, out of an estimated 16.5 million total firearm transfers in those states. 40 percent of this total—6.6 million transfers—occurred without background checks. Calculation based on FBI data. Available: http://www.fbi.gov/about-us/cjis/nics/reports/20130205_1998_2013_state_program_to_date_purpose_ids.pdf
 Daniel Webster, Jon Vernick, & Maria Bulzacchelli, "Effects of State-Level Firearm Seller Accountability Policies on Firearm Trafficking," Journal of Urban Health, July 2009. U.S. Department of Justice, Federal Bureau of Investigation, Supplementary Homicide Reports, 2011.  Centers for Disease Control and Prevention, National Center for Injury Prevention and Control. Web-based Injury Statistics Query and Reporting System (WISQARS) [online]. (2005) [cited 2012 Dec. 20]. SOURCE Mayors Against Illegal Guns