Reposted from the Snub Noir https://snubnoir.com/ Facebook page.
One of the issues with 9mm in a revolver is that the very characteristic that makes it reliable in autoloaders, its taper, works against it in revolvers. The 9mm cartridge tapers 10/1000ths of an inch over a case length of .500. Its taper gives it a high probability of extracting cleanly from the chamber of an autoloader immediately after firing. However, we don’t want revolver cartridges to extract immediately from the chamber. In fact, we want the exact opposite to happen. If the cases self-extract (move backward) far enough, they bind against the recoil shield. When binding occurs, the friction interferes with cylinder rotation. This is a problem.
The .38 Special, like most 19th Century revolver cartridges is cylindrical (.379) down its entire case length. The taper of the 9mm cartridge allows it to move backward in the chambers much more easily than a cylindrical case will.
Using moon clips mitigates this to a certain extent because the friction of all the chambers is exerted on the one cartridge that fires. It’s not a guarantee though, it’s ammunition dependent.
Moon clip .45s work pretty well because the .45 ACP has very little taper to it. Its taper is only .003 over the .650 case length, so it’s almost cylindrical like the .38 Special.