No obstructions I guess. This type of burst is very well described in one old Russian gunsmith book. By muzzle view I could say the gun barrels were very well honed and by overall condition and appearence of this particular gun I guess it's been honed in wrong way, when there are some areas of thin metal in barrels. When shot and wad comes through such barrel, the pressure behind the shot could puff up this weak area like rubber ball. Then process of blowing up develops like you can see on pics.
No extra loud sound, no huge recoil and probably the owner of this gun has dropped duck from burst barrel any way. All these can only prove there were not any obstructions in there.
I just wonder how this gun did not blow up long time ago?
