I think it's especialy challenging now that it is so easy to contact suppliers online and order things quickly directly- count in the cost of gas, sales tax, etc. If you were doing some other money-making activity at the same time (building guns, etc), it might work. When business is slow, you'd make more headway. etc.
The bigger variety of available components probably makes it harder now. You might have 30 barrels in stock, but a customer might want a B weight, etc, 44" long, not 42, etc. Once upon a time you'd carry some Douglas barrels and that would be it. Used to be you could carry Silers large, small, flint and percussion, and maybe 60% of the customers would be happy with that selection; another 25% wanting a Hawken lock. Now there are at least 20 good locks out there. Which should a small shop carry?
I would think you'd need $25K in inventory to get started.
Just some ideas. The idea that it might become an consignment shop is interesting. When I lived in NJ, there was a small ML shop in Neshanic- old Charlie Stone. I heard he has a small shop in PA now. Potbellied stove, doo-dads and gee-gaws of all kinds, some basic stuff. I sold 2 or 3 rifles, a couple horns, knives, etc. there on consignment before Dixon's opened up their first shop. Then the wider inventory and knowledge and Chuck's wonderful encouraging ways made it worthwhile to drive an hour and a half or two hours to go there at least once a month.