I got in some barrels made and I had one of the extensions unscrew. I asked the maker how that could happen - as the drawing I provided said 150 ft-lbs of torque. He said "Oh, I never use that much torque - makes the gun less accurate."
So he decided on his own to disregard my drawing and make it "his way."
That is one problem with the concept of gunsmiths - they know so much they tend to do things their way rather than to follow the drawing. For this reason I now work with general machinists and provide fully defined drawings.
So for example, I would be horrified to send a gun to a gunsmith and say "Please thread this barrel 1/2-28." This is because, rather than follow a drawing, they will ask for the mating device and just make them fit, but then the threads may be oversized and won't be interchangeable with all standard parts later.
Instead, I would do a complete drawing, and have a machinist thread it - and they will use a thread micrometer or thread wires to verify that it matches the drawing.
As far as a barrel vise vs a receiver fixture - today I removed a flash suppressor from a 9 inch barrel with an 8 inch rail. I had to use a receiver fixture. It should not be a problem, as muzzle devices are typically 30 ft-lbs of torque, and the barrel extension should need 5x that in order to unscrew. The pin, if made properly and actually tempered, should not shear.

