Two US Air Force (USAF) Security Police (SP) assigned to the 310th Space Group (SG) "Bombardiers" armed with 5.56mm M16A2 rifles don their MCU-2P gas mask during a simulated gas attack on the security forces marksmanship competition course, at Vandenberg Air Force Base (AFB) California (CA), during Exercise GURRDIAN CHALLENGE 2002. Guardian Challenge, a four-day space and missile competition, is hosted annually at Vandenberg AFB, CA to test the wartime readiness of Air Force Space Command (AFSPC) professionals