The ICP published a list of minimum barrel thicknesses for three types of steel classified according to the steel ISO specs. THe diagram gives dimensions for gauges 12, 16 and 20. The graphs are a little confusing and the minimum thickness surprisingly thin compared to actual wall thickness. On the other hand, a barrel is not built to a bare minimum thickness based on pressure and steel strength. A safety factor as well as future honing and bluing is taken into account when designing a gun.