Iraqveteran8888 put up a video of Ryan threading a Mosin Nagant barrel using a power drill and a special bit that is cutting the shoulder.
The bit has a pilot guide rod that helps keep the bit centered on the barrel. After cutting the shoulder Ryan can thread the barrel using a die.
Looks simple and easy. Just not sure what the guide rod is doing to the rifling inside the barrel. If the fit is perfect then the threading will be concentric. However the tighter the fit of the guide rod, the more it will rub on the rifling. If it is loose, then the rifling might not be damaged but then the threading won’t be concentric. I suppose it depends on what the pilote guide rod is made of.
And here is the fruits of his labor. It is so quiet.