At a vigil last night in Newtown, Connecticut, President Obama put last Friday's massacre of first-graders in the context of a year when multiple killings have become all too familiar. The latest of this year's multiple shootings killed six adults and 20 first-graders, none of them older than seven. Some call this the moment for action. Others warn about unintended consequences. With 300 million guns in circulation, what are the options?