head space and timing 

This is a military term related to ensuring the bolt and head space for the M2 .50 caliber machine gun are in alignment. This must be done correctly, or the weapon can explode during operation. It also means a person who's screwed something up because they failed to check what they were doing before they did it.

Lieutenant Smedlap blamed his mistakes on his platoon sergeant, but it was actually his fault.

"LT Smedlap blamed his sergeant, but it was really a head space and timing problem"

head space and timing by Soldier online October 23, 2009