The .380 has a slight taper to the case, with the base measuring .374" and the mouth measuring .373".
The 9x19 has a more severe taper, with the base measuring .391" and the mouth measuring .380". The only similarity between the two calibers is the diameter of the bullet, which is a nominal .356".
I'd suggest buying a good reloading manual, such as the Lyman 49th Edition Reloading Handbook. This would answer many questions for you.
Hope this helps.
Fred