Persistent claims have been made that expanding background checks to include any private transfers of guns would reduce mass public shootings. Yet, this is the first study to systematically look to see if that is true. In fact there is no evidence that these laws reduce the risk of these attacks. Examining all 47 mass public shootings in the US from 2000 through 2015, we find that states adopting additional background checks on private transfers see a statistically significant increase in the rate of killings (80% higher) and injuries (101%) from mass public shootings. There is not one mass public shooting that occurred over that period that these checks would have been prevented.