Or to be more particular, rapid-fire spud guns.
It seems that the only effective rapid-fire spud guns (which I have seen) are pepperbox style guns and harmonica guns. But to me, even these seem to have limited potential. (And most manually loaded breech loaders I've seen examples of don't seem all that rapid.)
The curiousity bit when I saw a spud "gatling" somewhere on YouTube. (I think it was via Wired magazine.) However it wasn't a true gatling, but just a big ol' pepperbox spud revolver. And from there I went to look at a true gatling gun example on YouTube and how its feed mechanism worked. And that's when the idea hit. The gatling mechanism needs a consistently shaped munition round to feed through for firing.
The problem with spuds is that they're inconsistenly shaped, and need to be rammed in, even for breech loading. Various balls are consistent, but the round shape with a lack of directionality seems to present another set of problems.


Now the thing is, these "cartridges" are totally inert. Because it's just a short section of pipe with a spud jammed in. Nothing more. The propellant (compressed air or fuel charge) will be provided by some means via the breech block. Thus your problems to be solved in that regard for rapid firing are the usual ones of compressed air valving or evacuating spent combustion gasses. Also once fired, the spent cartridges should be completely reusable.
But the neat thing about the cartridge idea is that you can explore various repeat firing mechanisms as used on real firearms (not just the gatling as I picture), because they are also designed around a consistent cylindrical round.
Also I thought of some minor refinements like a bevel on the inside of the breech part of the cartridge and a bevel on the outside of the barrel facing end, which would give some assistance in seating the cartridge for firing after being chambered. This is just to guide into some flanges that would hold it in place (as the way I picture it) between the breech and barrel during firing.
Good idea? Tested before? Other comments?
I did some google searching, but didn't see anything on this particular subject despite how obvious it seems. Thus my post here.