Would it be possible to make a harmonica gun in which the "magazine" is lightweight, easily made, and disposable? Preferably with the technology of the 1800s or earlier.
The harmonica part is the chamber of the gun. It has to resist the pressure of the black powder. That requires a significant amount of steel or iron for the harmonica slide.
The performance of the gun will suffer if there is a gap between the harmonica slide and the barrel. This allows hot gas to escape, which means
- the gas will not accelerate the bullet and
- the gas may burn the hand or face of the shooter.
This is a problem with revolvers and the reason why there are few which will take a silencer. So you need precision engineering of a solid piece of steel.
This implies interchangeable parts. In the early 19th century, each gun would be individually fitted. Around the time of the harmonica gun, the first gadgets with interchangeable parts were produced, and they were still rather expensive. While there were advantages to this from a repair and logistics viewpoint, it would mean that every harmonica slide would have to fit every gun. One couldn't simply file away edges until the slide on the workbench fits the gun on the workbench.