When someone mentioned "the world's smallest gun," I was expecting something along the lines of what Will Smith was packing in MIB. Surprisingly, that one was still too big for the SwissMiniGun, a real working pistol that holds the distinction of being the smallest rim fire ammunition available today.
No bigger, it seems, than a regular house key, the SwissMiniGun is a miniature double-action revolver, with complete mechanical parts as you will fund in a regular handgun. It comes with a holster that's designed to slot on a keychain (yes, it's a keychain gun) and is the product of Swiss jewelry craftsmen dabbling in the fine (ultra-fine, in this case) art of weaponry.
Measuring no more than 5.5 cm (2.16 inches) long, it packs 2.34 mm bullets that can be fired up to a range of 112 meters. There's no word on how lethal it can be, but a real bullet (even if it is smaller than a mosquito) traveling at high-speeds with actual gunpowder will probably cause some equally real damage. Despite requiring a hairpin to pull on the trigger, this should manage to deliver some amount of hurt.
Since it looks like nothing but a cute toy that you can get at the dollar bin, I doubt anyone will flag you for carrying this around. You can probably slide through airport security even while holding it in your hand. (we'd prefer if you didn't try that though).
The basic SwissMiniGun is available for $6,000 apiece, with special editions (like the bejeweled one above) going for as high as $50,000. Sadly (or maybe its a good thing), for US gun enthusiasts, it cannot be imported into the country although Canadians will be happy to know they can tote this thumb-sized firearm if they wanted to.