"IF" you were to cap off both ends of a shotgun bbl & pressurize the entire bore to the bursting point, obviously it would burst at the weakest point. This would almost certainly be the point where the wall was thinnest.
However when you fire a shotshell in said bbl a totally different circumstance exists. The pressure reaches its maximum inside the chamber & falls off rapidly as the shot moves down the bore, increasing the combustion area.
In relation to the pressure curve the weakest point in most shotgun bbls is the juncture of the chamber & forcing cone. As the charge nears the muzzle, then of course the entire bore will have a fairly uniform pressure but far less than it was in the chamber & it has been shown on several occasions even bbls of extremely thin walls will withstand this pressure.
It must also be borne in mind & obstruction does not uniformally pressurize the entire bore behind it, but instead creates a localized area at the obstruction having a high pressure. In the case of an obstruction the bbl will burst at or very near to the obstruction itself, irregardlesss of the bbls thinnest point. Many bbls which I have measured have the thinnest point back somewhat from the muzzles with the wall thickened as the chokes are approached. If an obstruction is lodged there at the beginning of the chokes the bbl will still either bulge or burst there, rather than at the thinner point behind it. This really has no bearing on firing a normal load through an un-obstructed bbl.
It is noted that when Bell blew up two Parker bbls, one steel & one damascus, they both finally gave way as pressures were around the 30K psi range. "BOTH" burst in the "Chambers", not down in the thinner sections of the bbls.
All of this has been well documented from way before I was born & I'm 73 years old, I didn't "Discover" any of this.
