The New York City Police Department possesses the capability to shoot down any type of aircraft if the city experiences a 9/11-type of attack in which commercial airliners or other aircraft are used as Kamikazes.
Police Commissioner Ray Kelly told CBS TV news program 60 Minutes that the force had the “device and training” for such an attack and “would have some means to take down a plane in an extreme situation."
Police sources later admitted the NYPD owns a Barrett .50 caliber machine gun, which is fitted to a police helicopter for extra security such as when a U.S. president visits the city. The .50 caliber is used in Iraq and Afghanistan mounted on U.S. military Humvees and helicopters.
And Chicago? Probably similar to this one: