I would expect, in the case of an obstruction, that the barrel would have blown just prior to that obstruction. I still lean towards an overloaded cartridge theory. I still think it may be worthwhile sending the photographs by e-mail to the Birmingham Proof House and seeing what their theory is. I'm sure they will have either seen it before or created it with their lab experiments. There's 200 years of experience there at the click of a mouse. At the very least they should be able to narrow down the search field. Lagopus.....