I havent been in that many gun stores, but I've visited a lot of fishing stores as well as computer stores, so I think I can speak a bit of what makes one of those local stores work, and why I would spend more money buying something there than I can get it from a bigger store, or online, since no smaller shop can compete pricewise with the big ones.
Like a lot of people have allready said, friendly staff. I often know a lot more than the sales people about what I come in asking for, but I never give that impression. Some people misjudge me as a know nothing and either try to denigrate me, or try to feed me some BS why I should buy what they want to sell. This is a really bad business move on their part. Usually I just leave, never come back, and tell everyone I know not to go there either. If I'm in a bad mood I might first respond by asking highly specific questions that they dont know the answer to, and do so in front of other customers. (Some of my friends find that really funny to watch.)
Bottom line, treat everyone with respect, and do your best to help them.
Knowledgeable staff, but if they dont know everything, they are willing to find out and learn.
If you dont know the answer, dont try to fake it. It's better to say "I'm sorry, I dont know the answer to that question, but I can find out for you", than try to pretend by giving a fake answer.
One of my favorite fishing stores years ago had the owners wife standing there a lot of the time. She was far from an expert, but she kept asking people what they used and how they liked it, so she could always tell you what the local experts used, and why they liked it. And she never tried to look more knowledgeable than she was. That was the reason I spent loads of money there, often buying things just on her recommendation.
Competitive prices. You probably cant compete with the big stores or mailorder businesses, but you should try to keep it as close as possible too. If you charge $250 for something I might find online for $200, the convenience of buying it from you, seeing it before I buy it, and be able to maybe get some tips along the way, will make it worth it. If on the other hand you charge $400 for it, it will make it harder to justify myself for buying from you.
Quality products. Only sell products you yourself think is good quality. I dont mean to just sell your own favorites, recognize the difference between objective quality, and subjective favorite opinion. But if you keep getting complaints on a specific product, quit selling it. This way customers will have confidence in knowing that the chance of making a bad buy at your place is really low, if it was bad stuff you wouldnt have it. (One of the computer stores I visit quit selling equipment to protect modems from lightning strikes, because too many people complained they didn't work. Instead they bought a load of cheap modems, and sold them at bottom prices to people that got their modems burned. They felt that was a better deal for everyone.)
This does not negate the other suggestions of selling cheaper guns, and cheaper ammunition. Just make sure that the cheaper stuff is still worth their price. As long as a $100 gun is worth the money you sell it for, I'd consider it "quality" in it's own pricerange.
I also like the idea of a hang-out corner of some sort. Just a coffeemachine, a few chairs, a bulletin board with local news, and maybe a video running a selection of shows would draw people just to come in and see what's new. This would make the store a regular visit for people when in the area, even if they dont plan on buying something that day.
A possible benefit of that is that when you do get a customer with a question you cant answer, there's a pretty good chance someone hanging out in the store can. I've done that myself on more than one occasion.
These things all have the same in common that the small store offer something the big store dont, which makes it a better place to buy from.