There's nothing new under the sun, they say.
A musket made in Nuremberg, Germany in the 1600's was loaded with 16 superimposed charges. It had two locks, a wheel lock and a match lock but only one trigger. In use, the first pull of the trigger operated the wheel lock which fired the 9 charges closest to the muzzle. A second pull on the trigger also operated the wheel lock and fired the next six, leaving the musketeer with a weapon still loaded with a single charge. To fire that, he had to either re-span the wheel lock or light the match lock and again pull the trigger. I'm guessing reloading took a while.
In 1856, a gentleman named George Kesling of Lebanon, Ohio, patented a superposed-load gun that was designed to load 12 rounds, one on top of the other. It had 2 percussion locks. The first (closest to the muzzle) load was fired using the front lock. The next 10 rounds were loaded with "perforated bullets" to allow the front load to ignite the next one back, etc. The last load - closest to the breech - was protected from firing when the rest went down-range so the shooter would retain a loaded weapon. THAT ball was not perforated. That last load had to be fired with the rear lock.
During the Napoleonic Wars, Britain's Henry Nock made a rifle called the Volley Gun, which was a 7-barreled .50 caliber flintlock with all 7 barrels firing at the same time with a single trigger pull. It was intended for the British Navy, to be used against enemy deck crews trying to board a sailing ship.
One of my source books has a photo of a 24-barrel pepperbox revolver. It's not clear whether the barrels are fired individually or all at once. The single hammer-multiple nipple construction suggests one at a time was the intended result, but we all know how well that works!
I'm guessing a holster for one of these would look a lot like a back pack, but I have a clear vision in my mind of some gentleman of the era pulling this thing out when he was confronted by a thug in some alley, yanking the trigger, and getting a chain fire. It would certainly discourage the thug --- probably permanently --- and also set fire to surrounding buildings. Mercy!
And of course, there was the 12-shot .36 caliber Walch percussion revolver with two concentric rings of 6 chambers each in the single cylinder and two hammers, two triggers, one set for each ring. Interestingly enough, the weapon was not significantly larger than an 1851 Colt Navy.