An expanded background check requirement for transfers of firearms seems to be the most popular of President Obama's gun control proposals, supported by nine out of 10 respondents in a recent CBS News poll. Yet it is unlikely to stop mass shootings, says Senior Editor Jacob Sullum, and enforcing it would require the sort of surveillance that has long been anathema to defenders of the Second Amendment, exposing millions of peaceful people to the threat of gun confiscation and criminal prosecution.