This is a small collection holder for spent (or unspent) rounds.

This particular set is for NATO Small Arms: https://www.wikiwand.com/en/NATO_cartridge

It's arranged based on the height model. (Photo is prior design based on diameter)

Note: The display photo was printed on fast, which ruined the text. The modeled uploaded here has larger, bold text and is meant to be printed on Normal.

If you want to print the casings:

50 Cal - https://makerworld.com/en/models/61349#profileId-63861

7.62 - https://makerworld.com/en/models/61350#profileId-63859

5.56 - https://makerworld.com/en/models/61348#profileId-63864

9mm - https://makerworld.com/en/models/146930#profileId-160281

5.7mm - https://makerworld.com/en/models/146946#profileId-160299