I’ve got four patents, but more importantly I’ve managed both R&D and collaborations with universities on intellectual property.
The first thing you need to do, before involving anyone else, is to create something like a business case for your invention. You need to research the gun industry to figure out where they are going and what their costs are, and create a model showing how your invention will either save them money or increase their market share. As mentioned, there is a big barrier to using ideas from the outside. There is even a big barrier to using ideas from another group in the same company, I spend 15 years in this environment. Your savings needs to be very significant - the legal costs and risks of adopting something from outside are pretty large.
Next, is your invention compatible with their designs and manufacturing processes? Incremental improvements are much easier to get adopted than major changes. If you are proposing a big redesign or something, you might as well not bother.
If you’ve really convinced yourself that you’ve got a case at this point, you need to find a lawyer and a sales rep who is involved in the industry somehow, and who has some connections to people in the gun companies, and who at least knows who the decision makers are. You’re going to have to convince him first. He won’t be easy to convince, because his salable commodity is his time, and you’re going to have to convince him that your invention will make him enough of a commission to be worth it.
BTW, before you even find this person, you need to run your business case by some trusted friends, friends who are willing to tell you you are nuts. Listen to them. Those with business experience will be best.
Congratulations on inventing something, but the creative part is the easiest part in these matters.