This is a simple low-cost pen gun. Its simple because you don't need any tools.
WARNING!!!! this is a dangerous tool. I will not be held responsible for any lawsuits that occur
Step 1: Make the Gun
now, what you need to do is to unscrew the top from the base. take the ink out. put it in the upside down top.
UPDATE: you need to stuff a wadded post-it note in the pen to avoid losing the spring
Step 2: How to Fire
to fire, push the pen ink down into the top. then let go quickly.
NOTE: the spring may fly out depending on your pen. If the spring falls out when you unscrew the pen, get a brand name like pilot.
I am using the cartridge as one of my mods for my heavily modded crossbow which has NOTHING to do with me