The tool after the threads is creating a blunt start/Higbee thread.
The number of roughing passes is a result of safe compromise. I found that no matter how well you are holding the tapered barrel, you can push it back into the chuck with a tool nose radius and depth of cut normally used to rough round stock. There may also be a few air passes because my program is written for nearly every muzzle diameter, rather than specific for the actual barrel.