The sandpaper mill tool is meant to be used on already turned barrels and excels at removing CA and squaring things up prior to assembly.  These sleeves are used to make the pilot stem approximate the inside diameter of the brass tube you are using.

The mill sleeves include 17 sizes, as well as the MDF storage board with pegs.  This is a finishing tool. The sandpaper mill tool is found here:  https://www.exoticblanks.com/sandpaper-pen-mill.html