Salary
Average Salary: $38,000
Expected Lifetime Earnings: $1,586,424
The whole making-money portion of this job mostly depends on your salesmanship skills, as well as the type of weapons you sell. Averaging around $38,000 per year, weapon sellers aren't exactly rolling in it (source).
On the bright side, though, it's one of a very few professions that work specifically with firearms, so there's a massive market tailored specifically to it's inventory needs. The gun and ammunition industry in the United States accounts for about six billion in annual sales (source).
Your customer base will be made up of sportsmen, hunters, collectors, and average Joes and Janes looking for a new item for home protection. How you sell to them is up to you. However, just because consumers want your product, it doesn't mean they're necessarily coming to you to buy it.
As with all sales jobs, the marketplace has the final say in how successful you are; you'll have to make all the right business decisions and know all the right terminology to keep your profits up.
If your mom's basement is the only place you can meet prospective customers (probably not a desirable location for your long-term business plan), or yours is just one of many discount gun stores in a square-mile area, your profit is going to reflect your lack of customers. With time, some business experience will help you figure out how to overcome those obstacles and increase your capital.
We're also assuming that your ultimate goal is to be in business for yourself. If instead you look at the sales counter of a Dick's Sporting Goods as the final destination, then an average retail rate of ten or eleven dollars an hour is what you'll be earning instead (source). What you'll lose in terms of income and responsibility will be made up for in schedule flexibility and discounts on tennis rackets.