the barrel? or what? why would you need to turn something into a pen? unless you pen spin. then you need to buy the pen with the correct barrel and take it apart.
get a beaker or measuring cup of water and fill it to an even point and record it. then put the pen in the water and record new measurement of water. subtract the new measurement from the old one and...
it is sometimes easy but most of the time hard. It depens on if you want to brake the pen led. If you do take a pen and put it on a ruler and draw on it until it is very deep. Then pull the pen side...