There is absolutely NO WAY for a barrel to breech midway down its length without a defect, a purposeful cut(abuse), or an obstruction.
Years ago, when Buckskin Report was still in print, John Baird showed pictures of a " hawken " style rifle that had blown out the bottom of the barrel. Someone had cut the dovetail for the barrel key TOO DEEP, and apparently either actually penetrated the bore, or was within so few thousandths of an inch of the actual bore that the barrel blew out, breaking the forearm of the stock, and sending splinters into the man's arm. There was a big broo-hah about whether this particular manufacturer could be trusted. I do not recall every hearing about another such barrel rupturing quite the same way. I have seen barrels that were victims of hobby builders who cut the dovetails too deep, but were wise enough to stop, and never shoot the barrel after discovering their error.
I have had access to a collection of barrels saved up over a lifetime of work by a gunsmith, who has since passed away, that suffered all kinds of ruptures. Most are shotgun or " smoothbore " barrels. They show ruptures at the chamber, in front of the chamber, half way down the barrel, and at the muzzle.
One is a classic 12 gauge shotgun where a 20 gauge shotgun shell made in down the barrel far enough to allow a 12 gauge to be fired behind it. The barrel is blown out at the side where the 20 gauge shell's base prevented the gases from going forward. You can easily see the stretching of the steel before the barrel ruptured, defining the manner and form of the obstruction.
Damage to the action, or breech of a barrel caused by all that pressure is NOT unusual at all. It would be surprising to see NO damage to the breech. The fact that this barrel shows that the barrel split all the way back to the breechplug, and then opened up to allow a separation at the plug just indicates that whatever the bore obstruction was, it was substantial enough to contain the powder charge gases, and did not move to allow the gases to go out the muzzle. It would be instructive if we had a good picture showing the actual point of the obstruction, from several angle, and the barrel out of the stock.
One can only guess what happened to this gun. I will advise others to be careful where you leave your gun at public events. Always check the barrel of your gun, if its been out of your hands and sight, with your ramrod, to see if there is anything in it that shouldn't be, including a load you forgot to shoot off before you took that break.
My best friend was a Registered Trap Shooter, with a 27 yard handicap. He was in a shoot at some club, and put his shotgun in the gunrack outside the club house, to wait his turn to shoot. He had made a habit of opening his action every time he picked up his gun, and checked the barrel to make sure it was clear. Someone apparently didn't like his competition, because while he was out of sight of his gun, they put a crush pack of cigarette down his barrel, and pushed it in so it was not easily seen when looking at the muzzle. My friend saw his barrel was not clear, used a club house " Stick" to clear the barrel. When he saw what had happened, he asked for a refund of his money, put his gun in the car, and drove away. He could have been seriously hurt, or killed, and, of course, a very expensive shotgun would have been ruined.
A retired police armourer told me that the shotguns held in the police cruiser racks were always coming back with cigar butts, cigarette butts, gum, wrappers, and other trash dumped down them. In his department, the officers were not required to clean the shotguns, and he had to do that. As often as he reported these matters to the shift commander, Nothing really changed. He always wondered what those slobs would do if they ever needed to use that shotgun to save their own or other officer's lives.
Be safe! :hmm: :hatsoff: