First of All Congratulations! You get to live your dream.
Now the work starts.
As far as the relationship with the shop upstairs, there are commonly 3 different ways these things are done.
Straight up rent-
You make a deal on your space, and you live and die by your efforts. Charge him for everything you do for them, pay your expenses, and keep your entire operation separate and distinct. Meet all the independent gunsmith requirements. You buy from him, he buys from you. Pay all your own bills.
If you give him a chart of fees for your work, and then later he wants to take 25 mountings or whatever off the rent, you're OK. But, landlords tend to expect to buy at the wholesale price (hence the chart beforehand). They hate to part with cash.
Split the labor-
You allow them to sell all parts, and pay all the taxes, and you vend service to them, and split the value on an agreed apon percentage. Advantage is less paperwork for your efforts. These agreements usually end up with both parties un happy. Issues such as who's work get done first around deer season, How you get your cut etc, are always thorny. When it's slow for him, he might be resistant to cash you out at the end of the week. And I stress cash you out entirely, every week. But, they will probably want cash for rent, because "You are using my utilities!" That always seems to pop up as time goes on. Expenses aren't magic, they cost. So, have a clear agreement on them. Otherwise, friction ensues.
Direct Employee, limited hours.
The third way this can be done, is as an employee of the shop. It's like having two jobs really. You start with a pre-agreed apon chart of what you get for what you do. You work exclusively for them from 11-4, and do all your private work outside that bracket. This scenario meets labor dept standards, as long as you are making at least minimum wage. The problems arise when the cash register tape says the smith is making more than the guy with all the business risk. And of course various taxes and paperwork for what you do privately.
Companies routinely want to renegotiate the commission schedule after they see who's making what off it.
I would be very surprised if your landlord doesn't want a taste of your private commissions though. Especially if there is some vagueness about origin of work. Are you directing work away from the shop as a way to put more in your pocket? These are all questions and issues to hammer out before you fire up the jets.
FWIW, You can replace gunshop with: Beauty parlor, Shoe repair, leatherwork, glasswork, fingernails, eyewear, lawnmower repair, furniture assembly and repair, bicycles, motorcycles, snowmobiles, tool grinding,and on and on. The idea is based on sharing of costs, and the craftsperson supplying an attractive,add-on revenue to the existing shop, without the existing shop having to start doing something new they are unfamiliar with. Or don't want to invest in.
But if you are clear up front about how you want the deal to work, it can work. These arrangements work everyday for people all over the place.
It seems hard at first, but trust me when I say, make a clear deal both of you can live with up front. And then get your duplicator going cause I have some clunkers that need buttstocks.
