Wow, this stuff is starting to go over my head but nevertheless, I have a few more things to say (not really ideas!).
I've never owned one but apparently percussion revolving rifles were not that popular, or more correctly, not that common. Given the relatively short period of percussion firearms, far and away most of them were produced for military use, a minor war occuring during the period, and perhaps they were too dear for the military and they had other competiton already by then, from cartridge firearms. However, much that might be true, I'd have to say that one would have to stick with the harmonica block (as a charging device) with built in percussion nipples, or otherwise you wouldn't have a muzzleloader. Strickly speaking, however, I don't know that you would absolutely have to use black powder just because it's a muzzleloader, provided it is built to withstand smokeless pressures. But that might be cheating and wouldn't be as much fun to watch.
I had mentioned that I was seeing as a shoulder fired weapon but you somewhere were thinking of it as a tripod mounted, heavier weapon. Naturally that makes a big difference. Without going into specifics, it should go without saying that when the weapon is heavier and even more so if the caliber, meaning bore diameter, is larger, it changes the dynamics of the whole operation. For one thing, extra weight will tend to dampen the recoil, which is a good thing, for it looks like a front-end loaded harmonica charger primed with percussion caps would take a lot of thrashing about and still be reliable. Both the cap and the charge and bullet have to stay in place. So that lets out any form of recoil system to make the thing work. One probably couldn't think of it in the same way as the Webley-Fosbery. Back to the drawing board.
A gas system is about all that's left, I guess. But there's hope but it looks like a fresh design is in order.
There have been gas operated systems utilizing something other than a box magazine. The Lewis Gun comes to mind. It had a rather slow cyclic rate (as a machine gun) but the idea here is to come up with something that will make the thing work without a violent action. A few (automatic)guns, even made in some quantity, were known for having a violent action. No, I think the real challenge of this project is to devise a system that will operate semi-automatically using the proposed charging system of a harmonica block.
Well, that's it. I'm all out of ideas except that I'm starting to think of accordians and I'm sure there's something useful we could come up with along those lines.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
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