So a cursed thing people did with guns during the percussion cap era was 'superimposed charges'. Basically, you load powder, and then ball, and then powder again, and then another ball. You have a chamber with two touch holes that go to different points in the chamber, two percussion cap nipples, and typically two hammers and some contraption to make them fall one then the other. The lead ball acts as a seal between the two powder charges, and bam: you got two bullets out of one barrel. People built ten-shot revolvers by using this principle.
Now, this never really caught on, because it's so obviously stupid and unsafe, but people did all kinds of stupid and unsafe things. So consider this: AU where the gun of choice in the old west is not the Colt revolver but a gun with a short barrel and a loooong chamber that you'd stuff full of five or six powder charges one after another, complete with some ratcheting contraption moving the hammer back and forth over all the different percussion cap nipples.