I recall reading a test done by I believe it was Guns & Ammo once on a particular model of .50cal muzzleloader. They tried different amounts of increasing powder charges & number of balls. Finally they filled the barrel half full of powder& then filled the remaining half with balls. Upon firing everything was blown out the barrel with no apparent damage.
As a last resort they dropped "1" regular powder charge & settled it in the breech & then pushed "1" ball half way down the bore. Firing this gave them two short bbls rather than one, blew it half in two.
Essentially no shotgun chamber made could stand the strain of even an ordinary powder charge as a "Closed Cell" pressure. What allows them to be safely fired is the movement of the shot charge increases the volume of the combustion chamber, thus preventing the pressure to "Max" out. This 12/20 combo obviously has a lot more weight to move so pressures increase dramatically. In your case they just didn't reach the bursting point. Whether or not the 20ga shell fires also has an affect on the pressure.
An obstruction is a totaly different circumstance. The sudden checking of the moving powder gases creates a localized high pressure point at the obstruction. If the obstruction has suficient weight to check the charge it matters not if it seals the bore.
