A U.S. Air Force Airman assigned to the 633rd Security Forces Squadron prepares to enter a building during active shooter training at Langley Air Force Base, Va., Oct. 28, 2014. During the training, Airmen used simulation rounds in their M-4 Rifle and M-9 Pistols in order to make the scenario more realistic. (U.S. Air Force photo by Senior Airman Kayla Newman/Released)