Develop a good relationship with Brownell's in Montezuma, IA. They sell most of the gunsmith tools, gun parts and supplies needed. Get a line of credit with several gun distributors. Don't try to get a big inventory of guns or parts. Customers will wait a few days if the price is right. Most guns and parts are shipped next day air or second day so you can get them pretty fast. Specialize in at least one discipline such as 1911's, AR 15's, DA revolvers, SA revolvers, etc. Generalize in all other disciplines. Don't invest a ton of money in shop equipment such as milling machines. Don't be afraid to say "no" to a customer if you don't have the equipment or talent to do a job. The fastest way to gain business is to do the best job you can and charge accordingly. The worst is to do poor work or over charge. Word gets around. Always charge for your work. Customers will expect freebees and will start digging stuff out of the closet if you dont. Get a good accountant and listen to them about cash flow and other financial issues. Ive seen very talented gunsmiths go down the tube because they didnt take care of their financial obligations.
I ran shops for over 30 years and always made a decent profit. Gun work comes in cycles. Either you're swamped or its dead, seems there is no in between. Be financially prepared for those dead periods. I could go on for days on the subject. Good luck and dont give up!