Gunsmithing would be interesting to see come up. I believe, after reading the earlier text, someone was under the impression that guns are rare and shouldn't be distributed as such. I believe the only realm where guns are taboo is Barovia? And that's a choice to keep things regressive there by the gubbermint. I used to have a character who would go to Port, buy a bunch of guns, power, and bullets, then sell them in bulk to randoms in Barovia. If anything, that was easier than I imagine gunsmithing to be.
But, let's say, Gunsmithing due to it's regional knowledge (And I mean, there aren't many, if any, DnD setting in which Guns can be smithed except for maybe Eberron or Gothic Earf) that you would need to have a specific token or item from an NPC, or DM that would permit you the knowledge necessary to understand how to make a firearm. There can even be beginner's pistols that are made that... Well, to be frank, would explode a lot more than normal. This token would interact with the crafting system to unlock a certain level of the blacksmithing tree, and the components of putting it together would require a mixture of blacksmithing for the barrel, and woodworking of some form for the handle and construction.
As far as mechanical limitations, there are rapiers in the game with pistols built into the dome of it, and I've had on one of my characters, a DM custom-make a firearm on an object other than a pistol. If a DM could apply a firearm property, surely there's a method in which to apply said property to essentially spawn a new class of items. This is a tired, 3:00 AM thought, but...
Carpenter makes.. Wooden handle, and construction.
Blacksmith makes a barrel.
Use the two together, you have a firearm without a mechanism.
Something fitting makes the flintlock mechanism, I'm not sure what craft would really fit.
Once the mechanism and firearm without a mechanism are used on eachother, it spawns... Something like Poorly Constructed Flintlock Pistol of which is pre-set in the toolset with properties already applied, and it just dumps the item in their inventory. Bullets can work under a similar principle.
If I'm not mistaken from past experiences, that's do-able, isn't it?
P.S.: Perhaps this would make Pistols much more relevant by giving players the ability to make decent, enchanted, or generally all-around swell end-game better pistols, or just ones that can more closely follow their progression. This would allow people to enjoy, or focus on them more without it being just a gimmick. Doubly so, considering they just expanded the crafting system to new levels for all of the metalworking and herbalism stuff, this would be kinda nice.