This missing data has little or nothing to do with it. America’s mass shooters aren’t getting guns because their disqualifying records have yet to be submitted to NICS by the states. They’re passing background checks because they are legal buyers under our badly outdated categories for prohibited buyers (1968). The only exception to that is Dylann Roof. The rest of our mass shooters are “Good Guys with a Gun” by the definitions of gun fetishists.

All that said, the primary reason for states not submitting disqualifying records to NICS is Printz v. United States, and that is entirely the NRA’s fault.