Lots and lots of guns have folding stocks, but the idea here is to find those weapons which will not function if folded.  There are a few.

The KB-ST GNOM.  A prototype which never went into production, and chambered for the 9x18mm Makarov round.

(Thanks to Tony for the heads up!)

The MGD PM-9 submachine gun, a French offering chambered for the 9x19mm Parabellum cartridge.  Seems that several thousand were made between 1954 and 1955, but they were more expensive than other subguns available at the time.  Considering that there were plenty of cheap surplus weapons available of all types that soon after WWII, and that the PM-9 design offered no advantages when the gun was being used for its intended purpose, the existing guns were seen as little more than pricey novelty items.

An example of two velo-dog revolvers.  Extremely popular in the late 19th Century as bicycling became all the rage, little revolvers such as these were designed to be used as protection against enraged canines.  Small, light, and virtually snag free, they also featured a trigger that would fold up into the frame when not needed.

The biggest problem for such small guns was that they were usually chambered for a proprietary cartridge, the 5.75mm Velo Dog.  Essentially a .22 caliber bullet perched atop a very large case, the Velo Dog round looked impressive indeed.  Pity that the ballistics were awful, with a low muzzle velocity and a pitiful amount of energy delivered to the target, which indicates that the designers had the impression that only lapdogs would chase people on bicycles back in the day.

Still, the trigger folds.  That is why it is here in this list, although I would agree that something as insignificant as a folding trigger really doesn’t qualify.

The famous Apache revolver, a French invention that was popular in the 1860’s.  (The images above were taken from the Wikipedia article.)

The idea is to provide an all-in-one package for defense.  When folded, the handle makes for a set of brass knuckles.  A knife blade can be deployed if going stabby is called for.  And, of course, there is the pepperbox revolver to shoot your way out of trouble.

I have written about this gun in the past, and any mention of the firearm usually brings a flurry of emails from people asking where they could buy one to be used for their own self defense.

I always reply with a personal note, stating that the last such gun was made about 150 years ago.  Surviving examples are terribly expensive, with even the worn and uninteresting specimens being sold for about $5000.00 USD at auction.  Finding one that was in good enough condition to still function as a self defense weapon would cost a great deal more.

The other problem was that the gun was woefully small, with even the 7mm revolver being terribly underpowered and weak.  The knife blade is a mere one and a half inches long, itsy bitsy when one considers that the average modern pocket knife boasts a blade fully two inches long.  And the holes for the knuckle duster are so small that anyone of even average girth would find it difficult to force their fingers in there.

The Apache was small not only because people back in the 1860’s were significantly smaller than they are today, but also because it was marketed for dainty rich ladies who were afraid that they might have to fend off a mugging.  This can be seen by the fact that almost all of the decently preserved Apache guns are adorned with gold leaf engraved with delicate little flowers and grape vines.

Isn’t that just precious?

CONCLUSION

So there you have it, a selection of handguns that can be folded for easier carry, but which have to be unfolded to go bang.  There are others, but I figure you should always leave your audience waning more.  If you have a suggestion, please be so kind as to mention your thought in the comment section.