Here's my 2 cents. I was in the retail and wholesale firearms business from '70 to '03. I worked in retail gun shop and for a gunsmith part time in high school for 4 years. when I got out of the service I opened my own shop on a shoe string budget. It was a struggle for 3 years. I watched the business change as the big retailers ate into the small retailers profits. A small shop can survive by finding a niche in the local business. I was primarily a wholesaler Ruger being my bread and butter as well as most all other firearms and accessories manufacturers. The retail profits in new guns are marginal at best. the Internet has greatly leveled the playing field (profits)in this area. People shop more and can do so from their home. there will always be a "cellar dweller" gun shop owner who will give the merchandise away just to make a sale. (never could figure out why someone would sell items at nearly 0% profit when it wasn't necessary, and there are a lot of them)I found in my particular instance that used guns, as most dealers know is where the profit is. But, you first must know the used gun business and it can't be learned by looking at a "blue Book". Right now gun sales are up most everywhere thanks to Obummer, and profits are good because some firearms and accessories are in short supply. the real profits are in the accessories. scopes, cases, mounts, holsters, etc. My shop had a "do you want fries with that" sales policy. the sales people had to try to tie in accessory sales with every firearms sale. To rely on profits from firearms sales alone is a recipe for failure.
Service....are you a gunsmith? Not a kitchen sink hacker with a screw driver and a work bench, a knowledgable and trained gunsmith or can you hire one? If this were to be my sole source of income. this would not be the business I would try to start in this econonomic climate. With one exception, I believe that a small gun dealer can make a good part-time income by taking his business on the road i.e. gun shows. I did 30 -45 shows a year. Some were geared to promoting the wholesale business and some were strictly used high end focused on the retail market. To be competitive in that business, again, you must know the used business.
There is a fine line between being having too much new and having to tell a customer that I don't have it in stock but I can get it. I used to have dealers that kept very little new in stock but relied on wholesalers like me to maintain inventory and order as needed. gun customers tend to be hands on people. If it's in front of them they are more likely to buy.
I was always a believer in maintaining a healthy inventory of new stuff,(variety don't put all your eggs in one basket, you might feature one brand but a little taste of the other brands won't hurt), that was in demand rather than special ordering. Trades will naturally follow the new gun sales. Having a reputation of having lots of "stuff" at fair prices, good service and knowing what you are talking about usually spells success.
I could go on for a long time and I do not want to discourage you from trying but you must know that it is one of the toughest businesses to be successful in even on a small scale. I haven't even touched on overhead expenses. If you know the down sides to this business the upside is a lot easier. Remember when you get that FFL and your home is the designated place of business, you potentially put it at risk. You didn't say if you were opening this in your home or a separate brick and mortar store. ATFE has access, if you screw up, the yellow tape is around your house not a free standing incorporated shop.
I am retired now and I still enjoy shooting and hunting but I do not miss the 60-70 hour weeks or the road trips loading and unloading the truck at shows. I do miss seeing my customers they were great people and they paid my bills for many years.
I wish you the best of luck in your endeavors. It can be very exciting and rewarding starting a new business.