Drew;
I think that 12,205 psi would be the static pressure it would contain if capped at both ends & if it were of the uniform diameter at which you calculated it. Obviously if it were capped at both ends & a static pressure applied it could be no higher than the weakest point in the barrel could contain. Since in firing a shotgun we are dealing with a pressure curve rather than a uniform static pressure the entire barrel does not have to be able to contain the Max pressure. The forward, thinner portion can be burst at a pressure which is lower than the max chamber pressure. Indeed under controlled test conditions barrels have been fitted with the normal pressure sensor at the 1" mark & obstructed with the barrel being burst at the obstruction without recording any increase of pressure in the chamber.
