Hole sizes and such are arbitrary. make them what you need them to be.
-What I do is drill and thread the inside of the barrel
-thread the outside of the barrel --> for cap threads
-flip piece around so the back of the blank is out, cut off the end of the blank
exposing hole, tap inside of hole for barrel finial
-remove piece from lathe
-install mandrel to hold onto the barrel.
-put live center on tailstock up against the mandrel to hold the piece solid
-turn down the barrel, leaving only enough girth so the threads on the end are thick enough to hold the barrel end finial.
(I hope this makes sense)
Turn trustone (or wood) round, drill it out, then on the metal lathe, use a boring bar to get the inside diameter to exactly what you need. Glue pieces together.
If you don't have a metal lathe or boring bar, then find a drill bit that will work for you.
If any questions - I'll try to be more specific.
