Joe, I'm not too sure which diameter you want, mouth ID, mouth OD or butt diameters, but the slimline pen tip fits the OD of a .30 casing perfectly. If you haven't tried a bullet pen before, here's how I do it:
In order to get the pen tube to fit the ID, I use an aluminum arrow (2018=20/64 OD with a .018 wall thickness) to glue the tube into and then the arrow is usually a press fit (it can be tight, but a gentle nudge with a hammer does the trick)into the case. The best way I found to insure the tube remains straight from mouth to butt is to run a 5/16 drill bit down from the mouth to the butt and stop about 1/16th short of drilling clear through the case. There is a shoulder on the inside of the case that runs up from the butt about 1/4 inch so this will give your arrow "shim" a perfect alignment every time. You'll need to fabricate a holder of some sort that won't crush or mar the outside of the case while you drill the case out. I use a piece of wood scrap with a 1/2 hole drilled through it and then cut the piece in half so the two pieces can hold the case in a vise while you drill. I then come back through the butt end with a 1/4 inch bit to bore the primer pocket out so the pen mechanism will fit down through it. I don't bore completely through the case with the 5/16 bit because that cuts off too much of the case size and type that is stamped into the butt. Some people like to see what size case they have.
If you want to use a European pen kit, then a 338 Win Mag case works perfectly, but I use two sizes of arrows, the 20/18 and a 21something or other that the 2018 slides very easily into. I can't remember which size off the top of my head right now. The rest of the assembly is virtually the same except I use a 11/32 drill bit instead of the 5/16. Of course you could glue your pen tube up in wood and turn it down to fit the mouth of the case, but I've found the arrows to work well and are faster for me to use. Clear as mud?
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