It's my shell catcher so that you don't have to search for the spent shells after shooting. Fits perfectly my Browning BL-22. I believe it will not work as good on bolt-action rifles, but on semi-auto and lever-action it will do a good job. There is a little button on the side so that you can rotate it and take the shells out. You will need to "unlock" it using a hammer after printing.

Print with support only touching the buildplate, the model upside down as in the photo.

Watch the video to see how it works (in french sorry): https://youtu.be/5Bl_Wf4ke8A?t=204