As an upperclassmen Mech. E student, I'm looking into the future at career possibilities. If you are serious about getting into firearms design, you need to be thinking Bachelors of Science in Mechanical Engineering with an emphasis on design, and probably a minor in materials. But as the guys before me said, you will face some heavy competition (probably myself included). If I could go back and do it again, I'd start in a tech school somewhere learning how to run CNC machines as well as the older stuff they teach us on here at my school. You can take calculus and differential equations and physics as well as a lot of gen. eds there as well, so you won't really have "fallen behind". CAD skills will be necessary, but most design-minded people pick that up pretty quick, and at least at my school, three different CAD programs are required, so that won't be something you have to learn on your own. ProE, SolidWorks, UGS NX5, and of course the old school AutoCAD are all prevalent in industrial design, but firearms design probably has a preference, so find out which program you need to master to get that job you want.
The tech school guys I've run into here are a LOT more proficient as engineers, in my opinion, and they tend to get out of school sooner (inside of 5 years). I'm getting a minor in material science as well, because as anyone in the industry will probably tell you, that's where the next breakthrough will be. Ceramics and their composites are getting more useful as time goes on. Having the knowledge before going into the work force will most likely help you later on.
I would say getting to know a gunsmith would be a must. You're bound to benefit from the knowledge he passes your way, and you may get the inspiration for the breakthrough design you're after. Tour a manufacturing facility if you can. Talk to designers themselves. Any or all of the above.
And as the person before me said, Mechanical Engineering isn't easy. It has taken me 3+ years so far, probably looking at 5.5 when its all said and done. But if its what you want to do, it will be well worth it in the end. I wish you the best of luck.