I would agree with Geno on his evaluation that there was probably not an obstruction present and rather involved a possible choke or reduced diameter immediately if front of the burst area due to prior honing to remove rust or scale or perhaps even possibly some previous dent or imperfection. It may even be possible to confirm that by measuring the bbl wall thickness in several places. If it is found to vary it would tend to confirm that supposition as causative.
What dram eq. loads were being used when the bbl failed, Saad? Does your friend know or could he tell you? I would almost bet he was shooting what we would call flyer loads [3.25 DE + 1.25oz. shot] or heavier.
I would suggest that you & your friends invest in a good barrel wall tickness micrometer and learn its proper use. There have been some very good threads here on the topic, inclusive of suspending the micrometer to get the best consistent readings and modifications to assist keeping the anvil pressure constant and even some photographs as well.
The gun shown has many indicators of heavy use in its prior life and probably should have been used only with low pressure loads and not modern ammunition at all. That is making the assumtion that it was on face & tight and it is also ignoring the proof mark, on purpose. Your friend is a very lucky person to not have suffered an injury .. as were you, my friend. Older double guns, were built from mild steels and as a rule they should not be subjected to modern maximum pressure ammunition loads. This is an area where the use of pressure tested reloaded ammunition comes into its own as it is quite possible to load ammunition suited [proper pressure range] to the gun's vintage that should be safe to use, if the gun is in good condition and everything checks out.
