If you run Barlow's Formula for a standard .729" bore at a forward location having an .030" wall you get a burst point of 3,645PSI with that same 54,000 PSI steel. When the shot hits an obstruction a localized pressure builds up, at that point. If that local pressure exceeds the yield strength of the steel a bulge will occur, if it exceeds the ultimate strength a burst occurs. When the barrel bursts, the gasses are fred so pressure falls rapidly, thus never reach the pressure recorded at the breech upon firing. The load in the bore of course makes it a Sealed Chamber, But the load is Movable & Not totally fixed, thus not a True Closed Cell pressure.

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