I don't think any real steel barrel maker would have the broaches to rifle a 6mm bore as it doesn't correspond to a real steel calibre.
I also have a feeling that barrel rifling is something most gunsmiths wouldn't touch. Firearms barrels are very carefully manufactured extreme pressure vessels which have wads of metal blasted through thousands of times. I'd leave that to a company who can do real proof testing and make stuff from super reliable billets of metal free of microcracks or voids.
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