so, it seems like a lot of folks have trouble with the junctions between the horizontal and vertical parts of rocket mass heaters. it also seems like those parts are prone to failure if not built very well.
forgive me for not having a great handle on the appropriate jargon, but would separating the vertical burny party from the vertical heat exchanging part instead of nesting the former inside the latter be a reasonable solution? those complicated junctions would be replaced by simple 90° elbows.
some potential issues that come to mind:
to keep the cross section of the system constant, a simple round pipe heat exchanger would have a lot less surface area done this way, so much less heat would be shed and draft could be compromised. using the common barrel for a heat exchanger would dramatically increase the cross section and work more like a bell, which might shed too much heat. I imagine there would be a sweet spot.
it's been suggested that the exhaust hitting the top of the barrel and then changing directions 180° around the outside of the vertical burn chamber creates a sort of spiral flow that mixes things up and improves combustion at that point. if that's an important feature, it would have to be created some other way.