To answer your original question, have you considered direct mailing flyers about your products and/or services to FFL's? If you go to the ATF web site, you can find a fairly up-to-date list of FFL's, to include their mailing addresses. If I were trying to tackle a problem such as yours, I would start by printing out a copy of those lists, by State. From there, I would sit down in front of the computer and do some research. (FYI, the only way you'd be able to do this in a reasonable amount of time would be to have a team of people working together, as there are tens of thousands of FFL's in the Country.) I'd give each person on the team a list and have them do an internet search for the FFL's on their list. A fairly basic search can give you an idea of how busy they are, how varied their products lines are, and how up-to-date they are with new and emerging trends or products.
If they don't have a web site, don't have their FFL listed on Gunbroker, aren't affiliated with some of the larger online retailers, or don't seem to have any social media presence, I would eliminate them immediately. If they're not doing any online marketing, they're probably very small, derive their customer base from word-of-mouth, aren't too into emerging products, and likely don't have a lot of disposable capital to invest. While I was narrowing down the list to those with an online presence, I'd be making notes about the business, such as web address, quality of the web site, types of products/services listed, number of Facebook/Twitter/Instagram followers, and anything else I felt was important to note. If they're only listed on GB, they're probably only interested in doing transfers. A social media presence, in addition to a GB listing, tells me that they're serious about growing their business and reaching out to customers, both existing and potential. If their web site looks like a high school drop-out or your grandmother built it, I'd put them further down the list than somebody who is adept at using technology. Then, I would sort the list of potential customers by geography. I wouldn't necessarily want to flood a particular town right away. If you're approaching them to buy your product, you might find it helpful to let them know that you've researched their geographic market and found them to be the most desirable business within 50-or-so miles, thereby giving a limited time offer of exclusivity. Another source for research would be ARFCOM, particularly the many Hometown forums, most of which have lists of FFL's with whom people are pleased or not. Additionally, most States or regions have localized gun owners web sites, where local people discuss various FFL's. You would also want to look them up on sites such as Yelp.
Once I had narrowed down the list, I'd choose the first 1-2,000 FFL's on the list and send them a flyer. You ought to be able to have flyers made fairly inexpensively at FedEx Office or your local print shop. Be sure to put multiple means of contacting you on the flyers, to include your web address, e-mail address, toll free phone number, and snail mil address. Then, pay for bulk mailing through the Post Office. Mail the flyers on a Monday, give them several days to arrive, and then start calling them during the second half of the week. This is where your research team can be put to use again, as you're going to spend a lot of time calling 1-2,000 businesses. Many, if not most, of them are going to flat out reject you. Some will want to hear your ideas and sales pitch. A few may become early adopters. Others might need some time to think about it or do some research. Don't give up. Depending on the product, you might want to prepare some demo samples you can send interested businesses, at your expense, especially for ones who seem genuinely interested and whom you've already identified as FFL's with whom you'd really like to do business.
Oh, and don't forget to start pages for your business on Facebook, Twitter, Instagram, and other social media sites. And, be sure to put that information on the flyers, such as using the little icons for the various sites, along with your business' handle on the particular site. If you do, however, be sure to have a well made and informative page. Don't just create the page and forget about it. (Think about the research you do to find the list of flyer recipients. If their social media pages sucked, you moved onto the next one.) Post pictures of your product, particularly of it in use. Post information about the product, especially it's features and benefits. If you're going to be at a trade show or gun show, post it on your social media pages, so interested parties can know they can get a hands-on feel of it.
In addition to mailing flyers, I'd see if they had an e-mail address listed on their web site, GB ad, or social media pages, and would e-mail them a digital copy of the flyer, with a link to your web site. Why both? Some folks hate anything that looks like spam in their e-mail folder, so they'll delete it. But, when they get it in the mail, they might take the time to look at it, since it's in their hands. Or, they might set it down and someone else in the business might see it, decide it's worth researching, and later convince the boss to give it a try. Other folks will immediately throw out anything they get int he mail from an unknown source, but may take the time to read the e-mail, especially if they don't get a lot of e-mails. Others will read both. Many others will simply discard both. But, one key to marketing is getting the product, service, or business seen by them enough times that they can't help but be subconsciously drawn to learning more about it.
Anyways, those are just some ideas. Good luck.