What qualifications do you need to design weapons?

Answer 1

First, actual understanding of how weapons work such as training as a Gunsmith, Knifesmith, Armorer, Peace Officer, Soldier, Hunter, or Shooting enthusiast. This you can even teach yourself by reading books on guns and weapons. Knowing what failed and why is also important.
Second, an understanding of materials and manufacturing such as training as a Industrial Designer, or Engineering degree provide.
Third, a way to communicate your ideas to others a degree in computer aided drafting CADD, Design, or the ability to make it such as training as a machinist would provide.

From, an arms Designer

Answer 2

Well i am going for a phyiscs degree, then doing a masters in gun systems design, and that seems to be a pretty good choice.