Staff Sgt. Katrina Aide, 51st Security Forces Squadron combat arms instructor, engages the charging handle prior to checking the head space of a M2 Browning Machine Gun at Osan Air Base, Republic of Korea, April 4, 2012. Checking the head space is a procedure prior to a function check to ensure correct spacing between the barrel and bolt. (U.S. Air Force photo/Staff Sgt. Craig Cisek)