Don't know if anyone else has run into this issue but I have found when expanding the mouth for the bullet on a .380 ACP certain headstamps have a thicker wall just below the mouth and the standard RCBS die will cause it to bulge somewhat even if you're just opening the mouth barely for a new bullet. I've seen this issue discussed on other forums as well. I emailed RCBS about this and they sent me this new expander die free of charge and it works great. You can see from the pic that the front of the die (snout) is much shorter and doesn't cause the issue.
380 Mouth Expander2.jpg