20 bore shells wedged into a 12 will give this 'chamber blowout' damage when the 12 bore shell is fired behind them.

The story given with the shotgun in London, was that the muzzles were shoved into the water while bird hunting at Point Pelee in South Western Ontario, in the 40's or 50's.

Perhaps that gun's and this one's Damascus barrel/s behave differently than modern steel with this sort of obstruction.

That they would blow at the obstruction makes perfect sense, though and I have seen other guns with muzzles blown off (single shots) because of immersion.

With this gun, constantly loading and only shooting the right barrel could easily allow the wads and shot column to move forward, then when the left barrel was fired, the wads and shot acted as a blockage. The somewhat thin Damascus breech let go where is wasn't supported (sitting on wood, which is common) - on top and to the left.

That forward moving projectile/s, due to forward movement in the unfired tube becoming an obstruction, is why we warn about using slugs (non-tight fitting like patched round balls) in a muzzleloading double barreled rifle. It is one of the very reasons slugs did not become popular for shooting large and dangerous game until fixed ammunition (breech loading) became useable. Reminds me of a bear hunt North of Terrace back in the 70's. Walking along, I just happened to glance at Tom's rifle. pointed & said, "I wouldn't shoot that if I were you". When he looked, the nose of the TC Maxi-ball was sticking out the muzzle of his rifle.