US Air Force (USAF) AIRMAN First Class (A1C) Holly Hood (right) and STAFF Sergeant (SSGT) Sandra Sturkie with the 90th Space Wing (SW), F.E. Warren Air Force Base (AFB), Wyoming (WY), fire practice rounds to zero in their 5.56 mm M16A2 Automatic Rifle weapons, during a Guardian Challenge competition. Guardian Challenge, the world's premier space and missile competition, is a four-day event hosted annually at Vandenberg AFB, California (CA)