Another small pen for another writer: the barrel, at her request, is 12mm at its widest. I used an 11mm triple tap for the cap and barrel. I almost went with a #5 nib, since the Jowo 6 requires a final 8.5mm ID counterbore at the tip of the section, and paring the section down to around 10.2mm OD to fit into the cap doesn’t leave a whole lotta wall left. But it’s fine once the nib housing is seated. (This is why I prefer ebonite for sections- since I pare the section by hand, I could see acrylic crumbling under my marginally competent hands.