Please print this in fun colors and play responsibly.
This is technically a remix of HTMF Metal Pizza's laser cut
https://www.instructables.com/id/Semi-Automatic-Rubber-Band-Gun converted for 3D printing.
The svg file has been imported to FreeCad or Tinkercad to modify it. You can start with the svg, the FreeCad files, or the Tinkercad files to customize this for yourself.
You will need the spring from a pen and 4-40 x 1/2" screw and nut from your local hardware store.