Mechanical pen

This is a sliding mechanical pen that takes 10.6 cm pen refill cartridges and consists of 4 3D printed parts.
Check out my Youtube video for the full assembly of the pen and more details:

https://youtu.be/BK7duWTtU3A

Don't recommend resizing as the fitting clearance will change proportionally to the pen and so will not work, on top of that the refill will no longer fit.

Here's an affiliate link to some refills 10.6 cm long although you may prefer to try to find even cheaper ones.
https://amzn.to/2StDP7M