Important: Print standing up. If you print it laying down, the rings will snap when you put a shell in. Print it like it's displayed in the picture.

In the print, I used 0.08mm layers. You should use whatever you think is strongest.

PLA has enough flexibility and strength to hold the shells snuggly without snapping the rings.

Feel free to use the STL or Blend file to create an additional mount for your buttstock. There is enough room in between the rings to guide paracord to make a harness for your buttstock or vest. The shells will be snug and will not fall out, even when upside down.