To the editor: Adam Winkler claims that we can reduce violent crime by requiring background checks on private gun transfers. This just isn't the case. ("Why banning assault rifles won't reduce gun violence," Opinion, Dec. 11) Indeed, California, Colorado and Oregon — sites of the three most recent mass public shootings — already have such laws in place. The same is true of mass public shootings that have occurred in France, Belgium, Norway, Germany and other European countries. Peer-reviewed research by criminologists and economists consistently shows that background checks don't reduce any form of violent crime. Law-abiding citizens, not gang...