Very possible- then new King's semi-smokeless was loaded in shotshells by volume, same as BP, but produced higher pressures. That could do the old gun. In the black powder days, black powder was simply called gun or rifle, or musket powder, not black powder. The switch to smokeless caused many problems, injuries and sometimes deaths. Usually, at least in some areas of the West, hand loading was strongly repressed, or advised against due to the lack of consistency of the early powders. In many of them, each 'batch' loading for ctgs. had to be carefully developed as lot to lot differences were extreme. Greener's "The Gun (etc)" shows and mentions this.

In retrospect, considering the length of the blowout, seemingly slighlty longer than a normal powder charge, wads and shot would take up - & that the tube ahead of the blowout seems enlarged, ie; very slightly expanded, makes me hold to an obstruction theory. Of course, this is all theory not being able to speak to the person who was shooting it at the time. Would THEY know why- not likely, unless it was the shooting fish scenario?

Note also the expanded nose of the hammer, where the mouth of the hammer's fish nose is split further and blown forward from the gas coming out the "tube"(nipple). I do think a smokeless blowout, might also blow out the nipple, but being a reasonably good German gun, maybe not.

Due to what I see as damage to the barrel(expanded) forward of the blowout, along with hammer nose damage, I do think it was a BP load, that was not sitting on the powder, ie: an obstruction.

This can easily happen if well over sized proper wads were not used. This, and if the right barrel is fired and reloaded several times without the left barrel fired, can have the effect of loosening the wads in the left barrel(rear trigger). If they loosen enough, they can creep forward in the tube until there is an open space between the powder and the wads making an air space - when finally the left is fired -- BOOOOM - the shot charge and wadding (whatever it was - maybe wasp's nest) are then an obstruction, not a projectile which causes the breech to blow due air space between powder and load.

This creeping ahead problem is THE REASON NOT to use bullets in a muzzleloading double rifle, only tight fitting patched round balls. Always have your bullet or wads tight to the powder in a BP load - 1/10" compression, if possible, is about right.