Final answer:
The U.S. Defense National Stockpile Center is responsible for issuing instructions governing the security of weapons systems.
Step-by-step explanation:
The organization responsible for issuing instructions governing the security of weapons systems within their inventory is the U.S. Defense National Stockpile Center. This center has built up reserves of various materials that are essential for weapons systems. They stockpile these materials to ensure that weapons' builders have access to them during wartime or armed conflicts.