The Swiss mini gun, produced in Switzerland by SwissMiniGun, is considered the world's smallest working revolver.
The ammunition is 2.34 mm rimfire, also produced by SwissMiniGun.
There is a key ring holster that comes with the gun when it is bought and can be clipped to a belt loop.
Many countries, including the United States and the United Kingdom, forbid the importation of the Swiss mini gun due to how easy it is to conceal.
[1] SwissMiniGun is recognized by Guinness World Records as making the world's smallest working revolver, their model C1ST.