This is a million dollar question. I'm an entrepreneur, having tried (with successes and failures, mostly failures) many fields, and I'm trying this gun thing now.
I'd like to make a go at gun instruction. So far, it's not doing well in targeting beginners and women (my startup niche), but I'm at least getting ideas from dabbling.
With gun instruction, one thing you can do is get certification for Utah CCW instruction. There seems to be demand among gun enthusiasts for the Utah CCW because of the 30-state capability. The class I attended early this year to get the Utah CCW had 30 students, each paying close to $200. That's a tidy $6,000 for the instructor for one afternoon of work.
Also with gun instruction, I'm looking at the long-term to offer security guard instruction. In my state (Cali), security guards have a requirement for 40-hours of training to start, costing them $400, and annual training. And there's a big demand for security guards (but they don't get paid much). So with a viable business, you can train security guards. Because of the high number of security guards, new ones and old ones renewing their training, it seems to me this can be lucrative.
If you get 30 new guards to train each month, that's $12,000 per month; if you get 50, that's $20,000 per month, if you get 100, that's $40,000 per month, etc. Seems good on paper.