I do not believe this to be a typical 12/20 burst as the 20 will lodge in the forcing cone. This burst was out beyond the forend, too far for that.

Once the Ultimate Tensile strength of the steel is reached a burst occurs. I truly wonder how one tells how much more pressure was involved than that. As soon as that burst occurs the remaining pressure Runs Out the Hole in the Wall. There is ample & Conclusive evidence which has been collected for a century or two that an obstruction can build enough "Localized" pressure to burst the Finest & Strongest barrel.

I Eagerly await the verdict.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra