Good discussions. What I hear are several different scenarios for non-professionally stocked guns.
1). A farmer or laborer without specialized woodworking skills decides to build or restock a gun, because he wants to, or wants to save money. Open questions are how he drills and taps, how he makes the ramrod hole, and why he wants a gun like that.
2). A skilled craftsman, perhaps a cooper, carpenter, other woodworker, or even a blacksmith decides to build or stock a gun for similar purposes. This seems plausible and we have an example in Colonial Fowlers of a New England fowling piece made by a clock maker. Its a beauty.
3) a frontiersman breaks his gun and decides to restock it using his pocketknife, axe, hatchet, file, and whatever else he has handy. Unless it was burned in a fire, it seems a patch job would be the way to go here, and we've all seen rawhide and sheet brass repairs. The situation in the Boone account would seem rare, but since gun smithing was in his family, it seems plausible he could make something that fired. As mentioned above, I'd assume it got re-stocked by a pro gunsmith when he got back to civilization, which could be Fort Pitt or any other major British fort in the colonial era, or St. Louis in the mountain man era.