You cannot beat the apprenticeship system for how a future gunny is trained. This does not in itself mean they will be great as some will struggle to be mediocre no matter how they are trained. But this system subjects them to 200 years of past experience in every area of gunmaking.

In the US, there is no such system. The best gunsmiths are the ones trained as machinists first.

Our guild system is a bit of a misnomer. There are members of the guild who are just checkerers (but great ones). There are ones who do only metal work and those who do only wood work. Some I do not consider gunmakers. Some I do not even consider stockers. One big name fellow (thanks to his writing skills) seems to be merely a finisher. How difficult is it to take a standard classic stock, bondo it up to be "your" design, and have Darcy Echols or someone else cut and inlet it for some action in question. In my estimation, that makes the "gunmaker" only a finisher.

The old suggestion of "Take that gun to a qualified gunsmith" for whatever is wrong with it is difficult when it is a double. There are very few qualified gunnies in the US for doubles. Frankly probably less than a dozen.