Turns out that a shooting buddy who is an engineering professor also has a background in tank artillery. He says that you also get autofrettage in normal high pressure gunbarrels because the pressures exceed the material yield strength on the inside but quickly fall below the yield value through the barrel wall. This ends up having the outside of the barrel compressing the inside which is a good thing.
He gave me a formula for calculating stress in thickwall cylinders (think barrels) but I am holding on to it until he confirms his memory. Should be soon. But one of the factors is chamber pressure so we still need to be able to establish that before being able to mathematically analyze a barrel.
