Again - what gun manufacturer is going to set up a factory and only sell guns and ammo within a single state?! :-) I understand the constitutional angle but the idea is absurd.
Currently there are at least 115 major gun manufacturers in the US, and 31 major ammunition manufacturers.
Many of the smaller gun and ammo manufacturers make custom, specialty and reproduction guns and less popular cartridge sizes and loads. So the idea of production in and for a single state is not unreasonable nor unprofitable.
Gun production can currently be very small scale, because parts and supplies can be procured across state lines. If a manufacturer wanted to set up in just a single state, they would have to insure the entire vertical production process was in state as well.
They would likely make higher valued and decorative guns with custom engraving to clearly indicate the state it was produced in. Many would be sold as gifts or awards, so form is just as important as function.
Likewise, they would have to make it a point that neither the gun or ammo could leave the state, maybe offering a “buy back” scheme in case the owner moved, or otherwise needed to resell the gun or pass it down in inheritance.
Probably the biggest buyers of standard, non-decorative guns would be police departments in the state, as part of the tax abatement and other incentives offered to manufacturers to set up in the state. This would especially be attractive to ammo manufacturers, as police go through a lot of training ammo.