SAN FRANCISCO — It was John Zawahri's failure to pass a background check that prevented him from buying a firearm in California several years ago. So the 23-year-old obtained an "unfinished receiver," the metal piece that holds the critical mechanisms that allow guns to fire, and built an assault rifle himself. Last summer, he went on a rampage at a college in Southern California, firing about 100 rounds and killing five people before police fatally shot... (GunPolicy.org)