The Ring bulge which is visible on both barrels is certainly indicative of an obstruction. The burst in the bottom barrel though is not really a common appearance of an obstructional burst. It may well have been an obstruction, with a loose base wad being most likely, combined with a seam in the barrel which held until opened by the extra pressure from the obstruction. Thus the obstruction caused the ring & the seam then split. The split could of course have been brought on by a thin spot as Larry mentioned, but either way as the ring is present it seems likely an obstruction started it off.
I would concur the second barrel was likely due to the deformation from the first one creating a restriction. It is of course possible, though highly improbable, that both barrels had a base wad come loose at the same time & then ended up at near identical locations in the respective barrels.
