This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A very serviceable fountain pen can be made from two 38-72 rifle cartridges and a steel pen. Clean out the cartridges, fit a hardwood plug tightly in the end of one shell, and cut it off smooth with the end of the metal. Drill a 3/16-in. hole in the center of the wood plug and fit another plug into this hole with sufficient end projecting to be shaped for the length of the steel pen to be used. The shank of the pen and the plug must enter the hole together. One side of the projecting end of the plug should be shaped to fit the inside surface of the pen and then cut off at a point a little farther out than the eye in the pen. On the surface that is to lie against the pen a groove is cut in the plug extending from near the point to the back end where it is to enter the hole in the first plug. The under side of the plug is shaped about as shown.
Ill: One Cartridge Shell Makes the Fountain Part of the Pen, and the Other the Cap
The other cartridge is cut off at such a point that it will fit on the tapering end of the first one, and is used for a cap. The cartridge being filled with ink and the plug inserted, the ink will flow down the small groove in the feeder plug and supply the pen with ink. Care must be taken that the surface of the smaller plug fits the pen snugly and that the groove is not cut through to the point end. This will keep the ink from flooding, and only that which is used for writing will be able to get through or leak out. - Contributed by Edwin N. Harnish, Ceylon, Canada.