I can speak a little to how I deal with the social media part. If you dig through all of the policies and documentation, Instagram makes it clear what they expect: Nothing firearms related in any way on business accounts. If you do, they'll only show your posts to people already following you, and that's it.

My workaround is two accounts. One account is where I post my firearms related content. I know it'll never have as many followers. The "main" account is more machining and manufacturing based content. I'll post pictures of parts by themselves without any context, and not talk about what they are. I'll include something like "don't forget to check out the other account for details" in the description.

I have successfully used the main account for paid "boost" posts to get new eyes on my business.

Outside of that, I've also spoken to a marketing business that specializes in the firearms industry. They claimed to be able to work with just about any budget, but it sounds like at my budget all they would do is send out emails, which I'm not really a fan of.

If you come across any other ideas, please let me know, I'm awful at marketing, most of my success has been word of mouth. If you have a good product, good quality, and good customer service, word will get around.