Both very valid posts, above. One thing that should be mentioned is that most gunsmiths now have a speciality area--example, riflesmithing, pistolsmithing, restoration and refinishing.
I was an active gunsmith from 1988 to 1995. My speciality was refinishing and pistolsmithing. Doing good work is not hard--however, it IS very time consuming--and you'll want to definitely mind your P's and Q's here. Because the thing that defines a successful gunsmith from a failed one is quality and customer satisfaction.
I don't care how much you have in an advertising budget; 90% of your advertising is done word of mouth, from customers to potential clients. If you do quality work, you'll soon have a waiting list and you'll pull in the money; if you do not turn out quality work, you'll be looking for another job real quickly.
Hiding in plain sight...