Yes , diameter is the same, but you don't want to try 124gr bullets in a .380.
Unless you are talking about specialty ammo like Liberty Defence, 9mm are generally 100 to 150gr, with 115 and 124gr probably the most common.
.380 is usually 80 to 100gr, with 90gr being the fairly common.
I have used the same 90 gr and 100gr hollow based RN for 9mm and. .380 but, I wouldn't go over 100gr in .380
I would recommend investing in a reloading manual like Lyman, Hogdon, Hornady, Etc.
You'll see a lot of the information I just mentioned is found in these manuals.