Threading a barrel is done with a tool that is the inverse of the shape it is cutting. A threading insert is a "thread form". The challenge for shops bound by conventional manual equipment surfaces when changing from imperial to metric threading. The same 60* thread form is used. It's the pitch, or number of threads per inch, that creates the problem. A conventional manual lathe often requires changing over to a different gear set on the back of the transmission to make metric threading work. On a CNC its irrelevant. All one does is either change from Imperial to metric using a single G code (G20 for imperial / G21 ='s metric) for most or you just convert the metric value to imperial (mmx.03937 gives you inches, etc) and leave the machine in G20 mode. (how I do it as its just easier for me to think in inches)
If the barrel is a true blank, then there's no features other than the hole and the outside contour. Threading, chambering, etc is not done till its applied to a particular setup.
Just fyi stuff in case someone doesn't quite get how this crap works.