1) If it was something related to your  job and/or you developed it in the course of your employment, it will belong to the military (e.g. if  you are an armourer and you developed  a better clip for a gun in use by the military).2) If you used the military's facilities (e.g. a lab, workshop) to develop the idea.3) If you have signed any agreements that the military owns any inventions you come up with, that agreement is enforceable.Ignoring item 3), above (that is, assuming there is no such agreement) if you develop something off base or site, on your own time, not using military resources, and unrelated to your work, then you should own it; otherwise, the military would likely own the idea.