In the early 1800s personal protection guns were all the rage, jut like the 2mm guns marked as Femme Fatal. Large enough to be worn on most any finger, the revolver had to be manually rotated through each cylinder. They were sold in small oval shaped jewelry boxes. For a time the French company sold them as a matching set his and hers, and they were dubbed the Les Companions.
These pipsqueak rounds, 2mm and 4mm,just have 2-5 grains lead flying at less than 500-fps from a cylinder with no barrel propelled by a tiny charge of blackpowder. They generate less than 2 ft. lbs of force, which is less than most pelletguns today, and is about enough to penetrate an inch of ballistics gel, or one side of a tin can.
These rings provided a certain state of perceived security for those solo traveling and, at the end of the day, the round would do some damage at extremely close ranges.
The Femme Fatales were never made in great quantity and are such a novel item that they appeal to any collector of odd firearms as well as vintage gun guys.