Only a quick question, and not making a judgement. What are the characteristics of a ring bulge. Is there any thought as to why the interior of the barrel that's opposite the blow out does not seem to be pressure affected (on appearance).

The orange peel look of the plastic type deformation of the affected area seems clear. Even if not deformed to the same extent, I'd guess the normal appearing part of the barrel was subject to the same pressure that started bulge prior to it letting go.

There's also a good sized looking pit in the deformed area and signs that some machining may have been done for maybe typical reasons that cleaning up may be done. I'm only curious, and would agree that an obstruction might easily have happened.