The issue with the barrel threading for US based smiths is in how (mechanical) lathes cut threads. There's a series of gears that are linked both to the spindle/chuck, and to the lead screw, which drives the carriage with the cutting tool. The different gears control how far the carriage feeds with each revolution, which in turn works out how many threads per inch you're cutting. Since the gears each feed at a rate which is fixed by the number of teeth and size of the gear, you're limited to the thread pitches you can cut by the specific gears in your machine.
In order to change from inch to metric you need to A: have the appropriate metric gears, and B: swap them into/out of your lathe in order to cut the threads. Most gunsmiths either will not have the gears, or don't want to be troubled with the huge hassle of setting up for a one off job.
A CNC machine generally uses magnets to reference its position and spindle speeds, with independent drives for each moving part of the machine. While this in very versatile in that it can cut virtually any pattern you can imagine (and program), it can also be a pain in the butt to do a one off job. For instance the CNC lathe at the shop I work in can thread any pattern I want it to, but in order for it to do it I have to write a program for it, set all the tools up, test it, and finally run it. With a mechanical lathe I can just slap the correct tooling in the quick change tool post, touch off on the part, set feeds and speeds and be making chips in a matter of minutes. Between the faster set up, and huge cost of a CNC big enough to do barrel work, mechanical lathes are more practical for a gunsmith to have in their shop.
But onto what really matters, how does it shoot?
In order to change from inch to metric you need to A: have the appropriate metric gears, and B: swap them into/out of your lathe in order to cut the threads. Most gunsmiths either will not have the gears, or don't want to be troubled with the huge hassle of setting up for a one off job.
A CNC machine generally uses magnets to reference its position and spindle speeds, with independent drives for each moving part of the machine. While this in very versatile in that it can cut virtually any pattern you can imagine (and program), it can also be a pain in the butt to do a one off job. For instance the CNC lathe at the shop I work in can thread any pattern I want it to, but in order for it to do it I have to write a program for it, set all the tools up, test it, and finally run it. With a mechanical lathe I can just slap the correct tooling in the quick change tool post, touch off on the part, set feeds and speeds and be making chips in a matter of minutes. Between the faster set up, and huge cost of a CNC big enough to do barrel work, mechanical lathes are more practical for a gunsmith to have in their shop.
But onto what really matters, how does it shoot?