Updated 3/25 8pm CT should fit snug without resizing. Do a 13mm test print to confirm.
First draft, use at your own risk, leave any questions or comments to improve the design. Untested. Modeled with .308 win/7.62x51 should fit any .308 or similar. Modeled at 100% if too tight or loose change +/- 1% and retry. If it works I have 8 and 10 round available. If rounds are too close together increase % on the Z axis or adjust by mm. This one is 65mm the 8rd is 104mm the 10rd is 130mm, they are designed at 13mm x rounds. The rounds are 12mm.
Print standing up, should not need support, may need brim. I would print pla+ or better at 100% infill. You can print less infill to test it, you can print them shorter to test fit. 13mm, 26mm etc.
The author marked this model as their own original creation.