Any kind of engineering, or computer science, really. I imagine weapons design utilizes every kind of engineering somewhere and lots of programming. Pick something out of those fields you find interesting, be it EE, ME, ChemE, CompE, or programming.
You probably didn't come here for an argument on the morals of weapons design, but I must ask, of all the ways you can contribute to the country, you choose weapons design? The reality is that there are things our country, no, the entirety of humanity, needs way more than weapons, such as alternative clean energy. And in the end, if having the biggest stick, as it were, is necessary for survival, humanity will inevitably be doomed to a very tragic ending.