My feeling regarding wall thickness is that the critical area is the end of the chamber. Here are my standards:
gauge chamber end diameter minimum wall thickness
12 .800 .085
16 .732 .075
20 .610 .070
The measurement at 9 inches from the breech is for a bore gauge. There are ranges of tolerance of departures from these diameters, but I use the following as benchmarks for 12/16/20: .729/.662/.615
I don't buy a gun that has chamber wall thickness below these minimums, and I prefer bores that only exceed the bore dimensions by .005 to .010, depending on the gauge.
It appears that there are already plenty of opinions on barrel wall thickness ahead of the breech, so I won't add my two cents to that.
This is for what it's worth.
