Red Leg,With greatest respect, this gun looks like it was destroyed by a blockage - not by firing through a tightly choked gun. I have no idea the details behind these particular circumstances, but when a choke fails it is apparent in the last six inches or so of the barrel. In fact, this incident looks exactly like a 20 bore load was accidentally loaded ahead of a 12 bore shell in a 12 bore gun. Another similar failure was caused by a partial ignition where the wad failed to clear the bore on the previous shot. I assume from your comment that 20 bore guns were not available eliminating the possibility of the wrong shell. An out of proof load will typically blow either the chamber or shoot loose the solder and thus the rib.
The big Greeners were built like tanks for the export market. Is there any chance another factor was at work?
Your right! As I looked closer at the photos, this disaster would have had to been caused by a barrel obstruction. Perhaps a barrel cleaning item left within the barrel? Back in the early 70s, I reloaded both cardboard and plastic hulls in my Remington 1100 12 gauge. The only issue with the cardboard hulls, was after many reloads with them, every once in a while, the brass base would separate from the cardboard case upon firing. No big deal really. I would have to remove the barrel to extricate the cardboard hull from the chamber with my finger. If I remember correctly, I was able to load the cardboard hulls with both more shot and powder. I would use candle wax to seal the crimp on the top of the hull to waterproof the shell. I’m sure many of the much younger shotgun reloaders here have no idea what I’m talking about with using the cardboard hulls, but they worked very well overall.