Do realize that when we speak of "Pressure" here we are talking "Max" chamber pressure. Push a given load to the same velocity with shells have respectively 7k & 9K psi. The 7K load will beyond all doubt have as much or more pressure out there where you hand is. The pressure curve will be different but if the same results are obtained the same work has to be done.
Light shot loads with low velocities are certainly appropriate for the Flues. As long as it has adequate wall over the chamber area don't be concerned about any pressure up to at least 9K. When we trade off to lower chamber pressure we compensate for it slightly for the rest of the barrel. Generally speaking this is not of great concern, but there is a reason that shotgun barrels are made so much "Thicker" in the chamber area.
With the advent of smokeless powders gunmakers began Beefing up the breech area of their barrels, they didn't beef up the rest of the barrel. Black powder burned slower than the early smokeless & thus had lower chamber pressure but higher barrel pressure. Believe it or not, but the gunmakers actually knew what they were doing.
