Oh! Thanks for the link!
I wonder if a J frame cut for 38/357 moon clips would be a better host for a 380 moon clip with regards to cylinder mouth dimensions.
Turns out the Taurus moons are pretty cheap so I ordered a few to good around with. If for no other reason than to see the ballistics.
Interestingly from that thread, I also have a bunch of 9x18 Makarov ammo lying around....
EDIT: no love for the Makarov