Having financed more than a few small businesses in my prior life, and worked in a gunshop as well, I need to point out a couple of things.
One, you are NOT your own boss. This is the biggest myth about owning your own shop. Everybody that walks in your front door is your boss and you'd better treat them that way. The bank that finances you will be your boss. The supplier you owe money to is your boss. The town planning and zoning commission is your boss. And I imagine ATF will have a few suggestions for you as well.
Two, plan on two to three years before you take any money out of this business. Any money. Hopefully your significant other has a good enough job to pay the mortgage and buy the groceries.
Three, you will quickly discover friends you didn't know you had or ever wanted. Friends that think you should sell to them at cost. Because that's what friends do.
Four, you can't run a storefront operation on a 10% markup. It won't fly, no way, no how.
Retail firearms is a tough nut to crack. There are still enough gunshows out there to make your life miserable. If your within 30 miles of a WallyWorld you can forget about ammo sales or low end shotguns and rifles. Your dealer cost on an 870 combo will be more than WW sells at retail. I don't know what the northern Wisconsin market is like but in cental Iowa retail stand alone gunshops are getting scarce.
Go slow on this.