I've been there (the situation, not the shop). What the guy needs is a person to manage expectations for both customers and the gunsmith. Ideally a wife or side worker who can give both customers and the gunsmith realistic estimates based on work and parts availability. When you're running a one man shop, you want to be able to help everybody, but in reality you can't. Also, when you're focused on your work, it's hard to put on the customer service hat and be that person as well. You'll never get anything done. If you focus in the customer service, you're not getting work done. There was a study that said every time you're interrupted by a phone call, it takes an average of 15-20 min. to get back to speed on what you were doing. Not to mention the time lost on the phone conversation itself. Four calls and you've lost over an hour's work. Sometimes 2hrs. Then there's counter time, dealing with customers.
We analyzed this a long time ago when some of our staff was complaining about production times. Our group also fielded support calls. We were taking about 20 calls a day. After our analysis, we told management that if they wanted production at their desired timeline, they would need to hire 4 new staff. They shut up after that.
Then, if you completely focus on the work, you're not attending to customers and even losing new business. I had this problem as a contractor. You get focused on the job and not fielding inquires about future work. Then you finish the job and you're suffering down time looking for work.
Just sayin'