Im looking into buying a moon clipped 9mm revolver, I shoot TONS of 380 so it made me think. Only thing I can think of is it might be harder on the forcing cone. Any other concerns?

.380 and 9mm have a different case head.  Don't know if 9mm moon clips will work on .380.

I'd be more concerned about split cases.

Why would cases split?

The 9mm chamber is tapered to accept 9mm cartridges, which are quite a bit larger at the head than the .380, though they do taper down so that at the mouth, they are very close.  

This means that the .380 case could indeed potentially split when fired, and that gas has to go some where.

Even if the case didn't split, I doubt that you could reload it successfully.

Not some thing that should be done, except possibly in an emergency situation.

Because the .380 cases are slightly smaller?

I have wrapped a layer of electrical tape around .22 Long Rifle and fired it in a .22 WMR revolver with no problems.

OK thanks, I figured since the bullets were the same diameter the casses may just be different lengths. I honestly dont shoot any 9mm so I didnt really think about case taper and head diameter difference. Thanks guys.

This WILL happen.

It would work but it probably would not have reliable ignition on 9mm moonclips, but it might.  It would be harder on 380 brass but would not split it.  It has been done before.  Look up Medusa M47 Multi-Caliber Revolver.  It could fire the follow cartridges all from the same cylinder without changes.

https://www.youtube.com/watch?v=UaNoDR1MMCQ

9 x 17 Corto
.380 Revolver
9 x 18 Ultra
9 x 18 Makarov
9 mm Browning Patronen (long)
9 mm Glisenti
9 mm Largo
9 mm IMI
9 mm Mauser
9 x 19 Parabellum
9 mm Rimmed
9 x 23 Steyr
9 mm Win Mag
9 mm Police
9 x 23 Winchester
9 mm Federal
9 x 22 R Japonés
.38 ACP
.38 Super
.38 Long Colt
.38 Short Colt
.38 S & W
.38 Special
.356 TS&W
.357 Magnum

I have a Smith and Wesson 940 J Frame 9mm revolver.

We have shot 95 grain .380 ball out of it on several occasions with no issues and good accuracy at 15 yards. (Which is a good distance for a DAO 2" J Frame)

Firing .380's out of the 940 has been discussed on the Smith and Wesson forum and several of us have done it with no major issues:

http://smith-wessonforum.com/s-w-revolvers-1980-present/270463-380-acp-940-why-yes-yes-you-can.html

The only thing that I have found is that if you are going to shoot .380 and 9mm from the 940 in the same session, fire the 9mm first. The .380's dirty up the tapered 9mm chambers and make the 9mm's very difficult to extract if shot after the .380's. A good cleaning of the chambers after a range session with .380's has resulted in easy extraction of 9mm's on the next firing.

The cartridges are a little loose in the moon clip and you have to hold them in the clip while loading. The cases do swell in the 9mm chamber, but resized in my Dillon Square Deal B with no problem and no splits after 3 loadings. I kept the .380 cases fired in the 940 separate to monitor them. No problems so far.

I like knowing that my 940 will safely fire both calibers if one or the other is not available and some recoil shy shooters that have fired the revolver enjoy the 940 much more with the .380's than with 9mm. YMMV in any other 9mm revolver than the 940.

Very cool, thanks.

Ruger makes an LCR380