I recently purchased a plasma pistol from ThinkGeek (RIP Timmy) and i felt it needed a plasma cartridge to go with it.
i modeled this to match the size of the Think Geek prop. This is also my first prop I've ever designed.
you will need 2 end caps and 2 rings, ive added a flat and a textured version depending on what you are looking for (the flat one should print better).
Edit: i added a chamfer to help the parts fit together and also designed a part that can be glue on after for the end caps