The murky world of drills,indeed you could write volumes about mistakes,shapes,usefulness,possession value etc. Best to be aware and forearmed with calipers. The sad thing is our approach to pens should be governing or developing a strong engineering approach (measure twice).
Goodness when I first started everything it seemed dependant on the lack of quality in manufacturers and suppliers of kits,glues. mandrels now theres a subject to defy your imagination . Wow they vary in diameter and so on. I used to go to engineering shops and take tubes and buy long machined bolts. I fixed on 1/2 pens for accuracy. I bought my drills by measuring them with my calipers.
Never a dull moment really,half the fun of success. Chin up and enjoy the chase,the thrust,shove and experiment. I made my first lathe ,loved doing it learned on the way. I do like the advice so far. I have an item in my autograph book from more than 70 yrs ago
Love many trust few but always paddle your own canoe. Yes I still am over trusting but I have met so many inspirational makers and people.
Have fun I do.
Kind regards Peter.