This unique revolver invented by John Walch in 1859. It has two triggers and two hammers to fire two rounds that were loaded into each chamber. James R. Rummel explains how it works:
The secret to the extra firepower is what is known as “superimposed loads”. Basically, the chambers are loaded with a powder charge with a bullet sitting on top, as is normal. Then another powder charge and bullet is loaded on top of the first.
The reason this doesn’t lead to an exploded gun and missing fingers is due to the unique ignition system. There are two percussion caps for every cylinder.
The gun is equipped with two hammers, and two triggers. Both hammers are cocked at the same time, but only the right-handed trigger is squeezed to set off the first shot. Then the left-hand trigger is squeezed, the left-hand hammer drops, and the second bullet goes flying. Cocking the hammers again will cause the cylinder to revolve as per normal.
Percussion caps are supposed to create a spark to set off the powder. Notice the ring of nipples to the outside of the cylinder? Those are the caps that are set off by the right-hand hammer, the hammer you are supposed to squeeze first. They don’t have a hole which goes directly into the back of the chamber, but instead channels the spark down a little tunnel. After about an inch, the tunnel makes a left hand turn and finally emerges into the chamber.
The hope is that the extra inch traveled will mean that the spark from the right-hand trigger will set off the powder charge in front, which will send the first bullet flying down the barrel while leaving the second bullet and powder charge untouched. The left-hand trigger will cause the left-hand hammer to drop, which will impact on the inner percussion cap, and hopefully cause the second charge to ignite.
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Previously: 20-Shot Revolver
Lets not kid ourselves. Part of the reason this never caught on is that it still does result in chain fires and missing fingers.
Although this technology still gets trotted out every once in a while. The Metal Storm system loads sequential powder charges in a tube as well although it electronically fires them.