I have done it a new-fangled way that is not at all 'traditional' but got very nice results and made a lot of customers very happy. I admire the old fashioned ways and appreciate the work that goes into it, but have no desire to duplicate it myself.
I used ball end mills on a CNC vertical mill with a 4th axis rotary and tailstock setup with protection for critical surfaces as required.
Now, technically speaking, ER Shaw has a patent on spiral/helical fluting barrels. That 'pattern' is apparently patented to them. I don't know that they defend the patent at all, as you will find all sorts of people offering the service, and I've done it for personal firearms and friends.
There are all kinds of trendy patterns nowadays that look ridiculous, imo. Like the 'tribal tattoo' craze and "XTREME" shit... that sort of flavor. From the hex 'fluted' barrels to aggressive diamond patterns. I've seen some where stainless barrels were bead blasted to a smooth-matte finish, then the flutes milled in, to reveal the shiny raw stainless. Some of it is more tasteful than others.
Regardless, imo, the most efficient, consistent, controllable way was using a rotary and tailstock in a VMC. Typical setup wisdom applies in regards to support/rigidity, cutting forces, etc.