I have a 20ga barrel off a Beretta Semi Auto which was fired with a 28ga shell lodge forward of the chamber. The barrel didn't fail catastrophically but there is a slight bulge and that is at a relatively thick area just ahead of the forcing cone. It was donated by a member of this board to my Hunter Education classes and is now sectioned so the students can see just where the smaller case lodges and what can happen. Will the barrel rupture or bulge every time - probably not. We are not worried about everytime, only those which do fail and can cause injury to the shooter. Homeless showed you photos of an o/u which blew right at the chamber. That was either a really gross over charge with perhaps the wrong powder or with an obstruction just at the forcing cone. Now what could lodge just at the forcing cone and go unnoticed. Squib loads normally clear the shot and leave the wad about half way down the bore which is where a squib rupture would show up. I suspect it was a smaller gauge shell.
