After I learned that 9x23 was originally made from cut-down 223 cases, I once cut a 223 down to 9mm size, reamed the mouth slightly, loaded it, and fired it successfully. I measured the volume with a graduated burette and adjusted bullet seating depth to make up for case volume. It sized easily but took extra effort to bell the case mouth. It actually worked better the second time, after it was fire formed to the chamber. Extractor required tweaking since the 223 and 9mm rim are slightly different (223 is a bit smaller).
It was fun to do once, but the total time to do 100 would be ridiculous. If you're doing it so your 380 cases will last longer (yes, I'm sure they will last longer) it probably isn't worth the time unless you've got nothing else better to do. You know what your time is worth.