Pete,
I don't think the gun industry then was any different from any type of skilled workmen going from apprentice to journeyman and then to master/machinist/tool and die maker,etc.
The stages and training would be the same. You found a job starting at the bottom., maybe sweepig floors, cleaning machinery, etc. If you took an interest and asked the boss that you would like to apprentice in a field, then you would go through that process in the learning stages until you were qualified to go higher, and so on.

In the gun making industry,usually the apprentice and journeyman worked on the lower graded guns, while the masters worked on the fine grades and all the details that went into it.
Your skill determined how long to reach your rank and wages.

David