Well I might as well throw in my two cents worth!
First off, I don't think the use of black powder would gum it up to the point it would not work. Was the Welby BP? Can't remember. You would prob have to clean it between 'reloads' oh well.
I would make it so that the barrel and chambers would recoil on a bottom slide, that way there would be enough mass for stiff loads. And that way the chambers would not have to move relative to the barrel except sideways.
I don't see why the barrel to chamber gap would be such a problem, it isn't on revolvers.
As far as if it should be in this thread, sure why not it is BP and he was talking about loading the chambers from the front, right?
Also I would note that Colt made a very large caliber like .50 if I remember correctly revolver rifle. That might be a more compact easier to carry design for such a gun, and with a top strap as the Colt design of that gun had a cylinder could be changed out faster or as fast as the horizontal harmonica chambers array.
Practical? Who cares. Fun? You bet.
But on that note, my 58 carbine is VERY practical in the underbrush for hogs, just as good as any modern gun would be, except you really only have one shot. But OTOH that is all you need if you do your job. And it is very cheap to shoot, and a lot of FUN.
Also I think the case could be made that IF we cannot get ammo anymore someday (soon?) then BP could do good duty as hunting arms at any rate to save what we have for more serious use.
I personally would like to fool around with a paper cartridge sharps rifle, they could be loaded as a muzzle loader, loose powder breech loader, paper cartridge gun, or metallic cartridge gun! With a bit of a mod, it could also be fired as a flintlock. Fascinating. And it was powerful and accurate. -Ken