Just to clarify, the shell in the top barrel did fire when the lower barrel burst?
I believe the double ring bulge, which clearly establishes the presence of an obstruction, was 1. an object (base wad, tipped shot wad, or all/part of the hull) in the lower barrel causing a bulge prior to rupture and 2. the lower barrel bulge obstructed the bore of the top barrel....
I think the top barrel bulge is more of a curiosity than the bottom barrel burst. At the start of the thread, Rob noted the the top barrel bulge 'is very uniform circumferentially'. The other thing is that the upper barrel bulge seems to be a visible bit downstream of the lower barrel bulge, though the burst extends further. It may seem that if an obstruction caused the upper bulge, it was past the point that the lower barrel bulged before failing.
I wonder if bulges on both barrels may have been present or forming without being noticed. It just seems odd to have two bulges in a similar location on different barrels. If it wasn't the loads, were the barrels weaker for some reason or another in about the same place, forcing cone or back boring work? Just thoughts. Your explanations Doc Drew are always well thought through and supported.
Rob, thanks for sharing the incident, and I'm glad your buddy is doing okay.
