This model is based on an enhanced version (3 wall layers) of a Tactical Ballpoint Pen https://www.printables.com/model/412788-tactical-ballpoint-pen. I added a clip to it.
You should print with 0.15mm layer height, as we all know, this would provide the most layer adhesion. Print any material you would like. I tested it with PLA
Print one pen and one endpiece. You'll additionally need an M3x10mm HEX screw for the handle to screw in the side of the end piece. Set it together with a Parker Mine (Koh-i-Noor 4442 works nicely too) and spring. (Mine and spring are sold separately).
For more precise writing control, you can heat the tip of the pen and adjust the diameter of the hole around the cartridge for a tighter fit.
EDIT 07.06.2023: Added model based on Normal version (2 wall layers).
The author remixed this model.