"There are enough dimensional differences that I'm not sure it would be easy to load the moon clips or get reliable ignition".
I tried to load one in my 9mm J-frame. It was difficult, but doable. However, I don't see a point to it.
Two different experiences with .40 S&W in 10mm revolvers.
https://revolverguy.com/psa-40-sw-in-the-10mm-gp100/
The Revolver Guy article matches my recent experience with three different 10mm GP100s. When using the factory 10mm clips provided by Ruger, they don't position the .40 S&W in the right spot to get reliable ignition. Even with factory springs I had spotty ignition when using factory clips. I switched to TK Custom clips specifically designed for .40 S&W, and out of two different revolvers I have fired 1,178 rounds of various .40 S&W ammo, with one failure to fire during that time, which I believe was due to a high primer.
Re: the original post to .380 in a 9mm gun. As mentioned, you'd have the same issue with clips not spacing the ammo correctly to get reliable ignition, and in my opinion it's not really worth it. I am not a big fan of small 9mm revolvers, and would rather see someone get a .38 LCR and shoot wadcutters, or get a .32 Magnum.
I really like my three 9mm J-frame conversions.
Ideal round for the ideal pistol :-)