Burrard gave much insight into this pressure bit. In fact he laid out a chart showing, using normal bbl construction & a normal load of the powder in then current use, just how far from the breech it would even be possible to burst a bbl, barring a metal flaw or an obstruction. A flaw can of course allow a bbl to open at less than normal pressures, though generally not with the violence of an obstruction. An obstruction due to the sudden checking of the charge moving down the bbl creates a very high localized pressure spot.
Beyond this a normal bbl, once the charge has moved a ways down the bbl, could actually hold the closed cell pressure if it could be just brought up to this level. This situation does not normally occur though as if the bbl is plugged the ugly affect of the obstruction rears its head & if the powder is spread down the bore it is extremely difficult to get ignition.
Botttom line though is that as long as the metal is sound & there is no obstruction in the bore, it is virtually impossible to burst a bbl very much ahead of the chamber. Technically though, all things considered, the juncture of the chamber with the forcing cone is the weakest point in a shotgun bbl. Fortunately though max pressure has already begun to subside when this point is reached, so it is normally of no consequence.
