They are not rare but seem to command a high price, I did have a 6/8 shot revolver but that got sold off years ago, again that was around the 2mm cal.

You can still buy these complete with the pin fire cartridges, obviously not over here in the UK.

I came across a photo and a small old discussion on a ring pistol? 

I remember many years ago I think it was at Bisley a Belgium guy had one of these rings for sale and wanted £300.00 which then was a lot of money, nobody bought it but it was a talking point.

I have found a couple of Photo's of these one was called the FEMME FATALE a seven shot ring pistol, not sure of the calibre but I would have thought around the 3mm.

By the looks of it you cranked round a chamber ready to fire by pulling back on the small sprung loaded clip.

I assume you left it with an open chamber and only brought a loaded chamber into play when you needed, to prevent the thing going off while doing everyday things, it wouldn't have taken much for that thumb trigger to catch on something?

The  FEMME FATALE looks like the real thing, Not sure on the other, it looks like a modern repro (dont think they had cross head screws in Victorian times ;-) It may well work as who ever submitted the photo also put a pin fire cartridge next to it.

So my next project is to have a go at making one of these.