I suppose it would not hurt to disclose the brand and model of the over/ under. It is a Beretta Silver Pigeon II Sporting.
I think we can eliminate a "cook off". The AD round had a definite firing pin strike. Besides the 19 rounds fired before this was done at 30-60 second intervals between each shot. I don't think the barrels ever got hot enough that you could not touch them. They may have been warm but not blistering hot.
"Breaking the shotgun fully open" may have some merit. Since I was firing the top barrel only I would break the gun open enough to only eject the top round and I would catch the fired round to dispose of the hull in a waste basket at each station. So there is a good possibility that the shotgun was not fully opened each time it was fired.
The history of the shotgun is that it is 7-8 years old. I have fired no more than 3000 rounds through it. The shotgun has never been fully disassembled for a cleaning.
What gunsmith would you recommend for the examination/work on this shotgun since you now know that it is a Beretta?
Thanks for all the insight into this problem.
