Do't be fooled by the 2 brass tubes. You will get a trully "single" barrel pen as long as 15cm. 7mm drill bit is required. 96.3mm cross style refill.
Note: Do Not glue the short tube but use it only as a spacer inside the barrel when turning. Remove the short tube from finished barrel and press it to pen tip. Before replacing refill, twine a few seal tape outside of pen tip in order to pull out barrel efficiently.