Over the last few months, we've received a number of calls, emails and returns based upon a number of factors. As a certified reloading instructor, it appears that this item should be given a light review to ensure new reloaders are aware of the difference.
Both the 9mm and 380 Auto (and others like the 357 SIG) use the same .355 diameter bullet. However, although the bullets are the same (9mm caliber that is), they are NOT totally interchangeable. They will both fit in the brass case yes, however, even though it fits, does not mean you should use it. Normally, about 100 grains and less are used for 380 Autos and above that are used for 9mm's.
A good example of this is the Hornady 90 grain XTP bullets. You can tell by the label it says 9mm, however it is indicating a 9mm Caliberor diameter, not that it should be put into a 9mm case. On the other side of the coin, although it says 9mm, you would probably NOT put it into a 9mm case.
Always, always, always, check your current data reloading guide before purchasing bullets or powder, and ensure what you are purchasing will work with your intended cartridge.