Mayor Michael R. Bloomberg is getting involved in an election other than his own this November, thanks to an issue he has long cared about: gun control. Hoping to make a splash in the Virginia governor’s race, Mr. Bloomberg showed up here Monday to unveil a new commercial criticizing former Attorney General Robert F. McDonnell, the presumptive Republican nominee, who is a strong gun-rights supporter. Mr. Bloomberg was joined about a dozen people, including survivors of the Virginia Tech shootings, their relatives, and Tatyana Timoshenko, the mother of Police Offcer Russel Timoshenko, who was fatally shot during a traffic stop...