If the top was FULL choke originally, are you saying the bottom was IMPROVED MODIFIED rather than improved cylinder? If that is the case, it is my understanding that steel shot, of the sizes you mention, should not be shot through chokes tighter than modified which improved modified would be.
As to the cause of the bulge, it would be my guess that at least one shell had an occurrence of "bridging", where the shot sort of sticks together in a clump resisting the chokes compression of the shot column and resulting in the bulge. If your bottom barrel is improved CYLINDER it still might have been bridging but could also be something else.
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