There dose not seem to be any significant evidence of a ring bulge.Other possibile reasons for failure are; A flaw in the Barrel material, and/or a thin barrel wall coupled with an internal longtitudinal score mark caused by removal of a boring tool, which in turn would create a potential stress raiser.This possibility is supported by the longtitudinal line/run at the site of the failure.A failure of this type is pictured on Pages 205 & 228 of the "Modern Shotgun" Vol 3. by Major Sir Gerald Burrard.

Roy Hebbes