I've encountered this situation many times before on various pen kits and blank materials and it can be attributed to a lot of different things.
Everything has a tolerance. Most inexpensive drill bits are manufactured to the low side of their tolerance. The factors that drive this include an assumption of the tolerance, concentricity, and runout (TIR) of the drilling process which not only includes the bit itself but also the drill, chuck, the characteristics of the material being drilled, and the holding mechanism for the material. In addition, for precision machining, it is desirable to have the drill provide a slightly undersized hole so the more precision reamers can do their job correctly.
For example, according to a precision cutting tools chart that I use, a standard 1/2 inch twist drill has a typical manufacturing tolerance of -0.0077" to +0.002.
Even though most drill bits themselves are manufactured to the low end of the tolerance, the holes that they produce are almost always oversized because of these factors. Drilling a softer, more resilient material on a well aligned lathe using precise tools such as a four jaw chuck to hold the work and collet chuck to hold the drill bit instead of a Jacobs chuck, can provide a hole that is even smaller than the specified dimension of the drill bit.
That being said, my process is to measure the tube with calipers, then depending on how resilient the material is, softer woods, harder woods, plastics, stone composites, etc., and whether or not I am planning to paint the inside of the blank, I choose a drill that seems to be appropriate based on the drills that are just slightly larger or smaller than what the tube measured remembering that I can always make the hole a little bigger - but it is problematic to make the hole smaller.
For example, the Baron instructions from Berea specify 25/64 and 15/32 for the body and cap respectively. Note that I typically tried to keep my spread to around +/- 3% on the charts, but there are usually quite a few options in the metric sizes, 11.7mm, 11.6mm, 12.1mm, etc. Here is a snapshot of my chart that would fit your Barons.
Regards,
Dave
