The instructions call for 1/2 and 13/32 drills. They look a bit big for the tubes that came with the kit. If you've made the Desire, what was your experience?
As per the instructions that come with the same kit from Woodcraft but named the Hancock (http://www2.woodcraft.com/PDF/77D33.pdf), I use 12.5 mm for the upper barrel and 13/32 for the lower barrel. I've made a number of these pens and these bit sizes have worked well for me.
I used 1/2" and 13/32" drill bits and the tubes fit quite nicely. I used epoxy to glue them into the holes and there were no untenable gaps to deal with.