80 thou at the end of the chambers; and 20 at the thinnest point, provided that's more than half way down the bbls (there is almost no pressure after the shot passes the half way point, although there is some longitudinal stretching at the choke if the constriction is heavy). The issue with thin bbls toward the muzzle is if you dent them, the dent cannot be invisibly repaired. Guns don't blow up past the midpoint unless there is an obstruction in the bore, in which case all bbls will bulge.

doublegunhq.com
Fine English, American and German Double Shotguns and Rifles