To me it sounds like you're asking more of a philosophical question. It seems that you are worrying that a healthy, avid interest in a (currently) controversial activity can morph into an unhealthy obsession with guns. Bear with me here, all, I heard the keyboards firing up at that last phrase, but admit it...obsession with anything can be unhealthy. Life is about balance; the apostle Paul counseled "moderation in all thing" I second the opinions above about balancing your boys' activities with other interests.
When I think gun obsession I think of the guys living in their parents' basement while accumulating SHTF gear out the wazoo and the next thing you know the SWAT team is using a ram on your front door, looking for your little monomaniacal "terrorist". We've all seen pictures on the news about this very thing. You don't want your boys to be that guy and I understand completely. I had several intense interests while growing up, mostly cars, music and guns. The "obsessions" has left me with an appreciation of fine machinery of any sort...guns to me represent a mechanical art form. Deadly, yes, but any well designed and manufactured gun, car or musical instrument (hey, oboes can be deadly!) is a thing of beauty. Your boys see that beauty and appreciate it. The intensity of the interest may wane but that's to be expected. There are lots of things demanding our time and attention.
You're on the right track. Awareness of their activities is key, and being involved in their passion is a great avenue to influence their interests. Music is a great balance...learning it is like learning a language. It requires discipline to learn and that carries over into all phases of life, including firearms training. You'd be amazed at how many musicians are shooters and how many carry concealed.
As far as your youngest son's design aspirations are concerned, encourage them. There haven't been may great gun designers, but the ones that we have had have changed history. Browning, Stoner, Garand, Kalashnikov, etc...it only takes one. Encourage him to study their design histories and the societal pressures that produced those designs. His interest may change; expect that. He may generalize into other weapons development. As we get more comfortable with the tools and technology, I think our interest in making bigger "booms" grows. Humans are tool makers and somebody needs to design good ones.
Good work and good luck,