Just read the entire thread. The machinists I’ve known probably didn’t have papers. They knew/know how to do anything that could be asked of a job shop machinist. They have decades of experience and problem solving minds. Machinists and model airplane builders are a lot alike. The level of precision is different but the ability to solve problems is the same.
I would trust any decent machinist with or without papers to thread a rifle barrel to fit a receiver. I would only trust a gunsmith that builds accurate rifles to correctly set the barrel up, thread it to fit the receiver snugly while concentric to the bore and chamber it concentrically to the bore. I don’t consider most gunsmiths to be machinists and I don’t consider machinists to be gunsmiths. They are different skill sets that use some of the same machinery.
I’ll never be a real machinist but thanks to the contributors here and the people that keep finding new things for me to make every week my skills are constantly improving. It’s almost like I’m not surprised to hit my dimensions without sneaking up any more. That’s as much better equipment as it is skill, though.