Obstruction bursts don't usually burst the chamber of a gun. They normally occur when the moving load hits an unmoving object somewhere down the bore. This gun is burst at the breech, much more like an extremely high pressure burst.
What about the possibility of a 28ga shell being in the chamber ahead of the 20ga, can this set up the same condition as the 12/20 combo??. This is of course an obstruction of sorts, but is normally listed as a separate category of burst than an obstruction one.

Miller/TN
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