"//"Law of Conservation of Firepower: Any powerful weapon capable of destroying/defeating an opponent in a single shot will invariably be reserved and used only as a last resort."//
Of course it's the last resort; if the enemy has been destroyed, why keep firing?
This is like: When you've lost something, it's always in the last place you look."
Reminds me of this one I hear often here in Colorado - It's always the last run of the day that people break their legs on.