I believe that I got this from Shotgun World under the Reloading Forum. There was a whole story on pressures and psi and the gentleman that wrote it did a lot of homework. Makes sense to me.
"Almost all shotgun powder burns within the first 2 inches
of the chamber. A typical fast burning powder will burn
in the first 3/4 inch of the chamber. This is where
maximum chamber pressure is attained. A typical fast
burning powder like Red Dot might give a chamber pressure
with a certain load of about 10,000 psi at that point.
This is why a chamber is very thick on a shotgun barrel
back at the chamber and gradually becomes thinner as
it gets further from the chamber."
There was a lot more written.
3/4 : 10,000
1 1/2 : 5,000
3 : 2,500
6 : 1,250
12 : 675
24 : 338
He stated that every inch multiplied by two the pressure dropped in half.
