When I opened my gun shop, many years ago, I was in the heart of muzzle loading country. The plan was a muzzle loading only speciality store. Business was near zero. I learned several things very quickly. Trading is a big key to selling new guns. There are many-many more potential customers for modern guns than muzzle loaders. Gun buyers need to have confidence in who they are buying from (exception these days are the big discount stores with deep price cuts) and they want good service.
After I started stocking and trading modern guns my ml business kicked in. As customers got to know me they referred others to me and brought in guns for trade. I always made a point of letting customers I could special order anything (legal) they needed, including parts and accessories. I hired a gunsmith and that really brought in the traffic, and profits.
To compete with the discounters I sold under or over them but no way could I compete head to head with identical products so I didn't try. But customers would buy their low priced guns (in modern usually H&A at that time) from K Mart then come to me for accessories and gunsmithing. I loved K Mart being there.
If what you plan is only a side line maybe friends will get the word out and you will be able to sell the items you have on hand.
A good sign out front, getting around to shooting and gun related events is another good way to promote. But promote you must do.