I wouldn't imagine there's much of a market for small-arms design. It's an old-school, natural-monopoly sort of subject, one of those for which the slow, careful, bureaucratic model that Paul Graham (correctly!) condemns for software was originally devised.

So I'd expect three things to drive down the value of small-arms designers: the point that variable costs are much higher than fixed ones in small-arms manufacturing; the presence of bureaucratic inertia in the militaries and police agencies who represent the main customers; and the fact that the problem domain is very well-understood and has been for quite some time. The AK-47 is showing its age, but is still viable; the H&K G3, the FN FAL, and the M-14 are all around 60 years old (the M-16 is a newcomer at 50 years, and the "space-age" bullpups, the Steyr AUG and SA80, are around 30); and the Colt M1911 was designed before the First World War and, from what I understand, is still one of the best pistols on the market.

There are a few "cutting-edge" weapons that are just coming into use -- the Tavor and the H&K G36, most obviously -- but I think these reflect a change in the perceived threat environment more than an advance in technology.

So, in other words: I'm not sure that there's very much to _do_ in the field of small-arms design. There are some small, innovative companies in the field -- Calico Light Weapons Systems is an obvious one -- but the problem with being a small, innovative manufacturer of firearms is that you don't have much of a market. Even when Bill Clinton _isn't_ in office, private citizens will never spend the kind of money on small arms that governments do.

I want to emphasize that I'm coming at this from the perspective of an interested amateur, and my interest is more in infantry tactics than in the weapons they use to implement them -- so there may be factors here that I don't know about. I'd second the recommendations by hga and gexla: contact people who know more about this, but, I'd say, prepare to be a little disappointed. (Aerospace engineering strikes me as being a field which is still evolving, and thus ought to be more promising for someone with an interest in designing weapons systems.)

I would go for Mechanical Engineering if I were you.

Do you live in or plan to study in the US? Even if you don't, try contacting the National Shooting Sports Foundation (NSSF, http://en.wikipedia.org/wiki/National_Shooting_Sports_Founda...) and Sporting Arms and Ammunition Manufacturers' Institute (SAAMI, http://en.wikipedia.org/wiki/Sporting_Arms_and_Ammunition_Ma...). In Europe, the equivalent of SAAMI is The Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (C.I.P., http://en.wikipedia.org/wiki/Commission_Internationale_Perma...)

I can dig up a German small arms research organization out of my email archives if you're interested.

Check the career pages of weapons manufacturers.

http://www.heckler-koch.de/Apprenticeship