An UNOBSTRUCTED shotgun barrel is very very briefly "obstructed" by the ejecta, which very quickly begins to move in response to the gas pressure, which then rapidly falls.
This is exactly why I have so much problem believing that an overloaded (double charged, wrong powder, etc) shell, will ever burst the barrel wall several inches down bore. The pressure on that curve is down to ~2000 psi at 7 inches from the breech. That's less pressure than my JD tractors' working hydraulic pressures, and they are contained in some very thin walled tubing and run for thousands of hours at those pressures, including pressure spikes.
Until proven wrong with empirical data I will always believe a down bore barrel burst is caused by an obstruction other than the payload itself.
