20ga in front of a 12ga in an 870, no failure:
I don't think a locked breech(single/double) barrel gun will handle it as well; the pump action popped open because of the excess pressure. My Mossberg only pops open on very hot loads; even then it doesn't open to the point the shell is extracted... If you had kept it fully locked, I'll bet it would have fragged the chamber.
Consider this:http://www.youtube.com/watch?v=9sN8tyzCUS8&feature=plcp&context=C4548060VDvjVQa1PpcFOoZgf9KcOB1X7dyxXmyk_HSchQ7gE65AE%3D
2" of mud in the barrel of 870, no failure:
If you've ever examined pressure curves for a shotgun, pressure drops off dramatically in the last few inches of the barrel. By the time the payload reaches the obstruction, the pressures would have already peaked and won't usually raise the chamber pressure enough to rupture it.
.50 BMG from a 12 gauge (H&R?):
A 50bmg uses extremely slow burning powder, it will never reach full pressure in an overbore barrel such as a shotgun barrel. As you see in the vid, there is good amount of powder in the bucket.