If it is EDM I think they would have to have an electrode made from very dense graphite in the shape of bullet form with the lands and grooves in it, a male form of bullet and the barrel bore would be almost the small I.D. the EDM machine would be CNC controled with the twist rate figured at so many degrees per inch. I dont know that this is how S&W does it but the place where I worked for 41 years used this type of eguipment in punch and mold makeing. In 1988 they bought a company in Alabama that make the button rifleing buttons for Colt for the AR15's and M16's I had the tool room guys make me a .357 button with 1-10 twist to try to make my own barrels, they tic-tin coated it for me I reamed a barrel blank to the small dia. and tried to press the button through. Its still in the barrel, but Colt supposedly just pressed the rifleing reamer through the barrel bore. EDM would work with great equipment and skilled operators.