Pete, as has been mentioned, there's no such career as "warfare physicist." The people who design guns (probably engineers or something) and the people who design nuclear weapons are in very different types of fields, and there aren't any physicists in the military whose expertise encompasses all of these things.
If you want to design weapons, my guess is that an engineering degree might be your best bet, but I don't really know anything about engineering. If you're interested in physics and would like to make weapons and other cool destructive stuff for a living, you'll probably want to get a PhD in physics. The specific field doesn't matter all that much; it could be anything from condensed matter to astrophysics. In fact, one of the colloquium speakers at my department last year had a PhD in astrophysics, and did classified research on nuclear weapons at Los Alamos.
Because of the whole test ban treaty thing, America doesn't actually make nuclear weapons anymore. However, the government does need physicists to maintain our aging supply of nuclear warheads, as well as to do simulations for the design of nuclear weapons. I'm no expert on employment opportunities, but based on what I've seen concerning physicists in the military, your best bet would be to major in physics and engineering in undergrad (the engineering degree is so that you're employable if you change your mind about physics), and then do a PhD in some area of physics that you find interesting.
