OK, so after reading thru all the "developments" in this thread, I came up with another idea. Sorry, I have no drawing skills, or auto-cad to give visuals, but I'll try to explain the best I can.
Take the harmonica rifle design concept and take it into the 'crew-served muzzle loading medium machine gun"
(admittedly, I'm leaving out the fouling properties of BP, and probably a whole plethora of other issues, but hear me out)
Also, I'm using several terms very loosely to aid my explaination, don't hammer me if the terms aren't "technically" correct
Here goes, I hope this makes at least a little sense to someone
Take the harmonica rifle as a starting point, but rotate the "magazine" to the 12 o'clock position so it drops down, rather than moving side to side. Possibly even at a 45 degree angle, moving from 10 o'clock to 4 o'clock.
Either way, now add a reciprocating action similar to that of the Browning 1895 "Potato digger" MG. (I believe one of Mr. Browning's earliest autoloaders was a modified Winchester rifle using a gas operated lever-action)
The op. rod of the "digger" actuates a lever which runs along the "zig-zag" grooves in the "magazine." The action of the op. rod would push the "magazine" into it's next firing position. The action could possibly be aided by tipping to magazine forward, making it a parallelagram with angled chambers, instead of a rectangle. (think of the angle that .22 cartridges sit in a straight pistol mag.)
One variation could be to use a revolving cylinder instead of a box style harmonica "magazine"
Ignition would be using a Maynard type primer strip which would be advanced when the hammer was reset.
Mount it on a tripod. Crew it with a gunner to aim and fire it, and an assistant gunner who would reload the "magazines" similar to the Japanese Type 92.
Am I crazy?