[fred cook]

So, the .38 Special / .357 mag and the .380 ACP / 9mm are dimensionally close enough to be used in the same cylinder and fired down the same bore. Ruger, for example, makes a 9mm cylinder for their .357 mag Blackhawk. It works o.k., even though the auto pistol bullets are smaller in diameter.

The 9mm is usually a .355" dia. bullet, the 38 special/357 mag is usually a .357" bullet. In the middle is the 380ACP that uses a .356 bullet. I have used the 9mm bullets in the 38 Special case with good results.