I didn't cover insurance, an important part of a gunsmithing business. If you have a fire in your shop, and it destroys a quarter million dollars worth of Purdey shotgun, you better be insured. And you also can run into other liability issues. You do a beautiful trigger job - so beautiful that the gun fires accidentally and someone is killed. Or you fix an old Damascus barrel gun which promptly blows up and takes your customer's hand along for the ride.*
So insurance is damned important.
Another is knowing and understanding all the laws, rules and regulations that affect businesses. Some folks think all they need to be a gunsmith is a set of screwdrivers, a file, and an FFL. They don't know about business licenses, sales tax licenses, zoning laws, OSHA regulations, EPA regulations, etc., etc. (Why do you think few gunsmiths do tank bluing any more?)
Also, it is a darned good ideal to have an attorney on retainer in case you get into some kind of trouble with all those laws.
Another issue not often considered is location. I have been berated and insulted over this one, but here goes again. I strongly recommend you NOT run your gun or gunsmithing business out of your home, even if zoning laws allow it. There are two reasons, one serious. The less serious is that your wife will be very unhappy when your customers wake you at 3 A.M. to fix their guns so they can get to the woods or fields early on opening day. The serious is that if a bunch of drugged up thugs want to raid your store for guns, it is better that they raid your closed business premises than your home and put your family at risk. Also, have an unlisted home phone and don't tell casual customers where you live.
*Work with your attorney in drafting a release form that a customer will have to sign if he wants you to do work that could make a gun dangerous, like fixing an old shotgun so it will fire. Or just turn down any such work.