As a rule Holts notes anything below .020 as below recommended wall thickness. But most if not all British 2” doubles started out below .020. Have the walls measured and decide. If the bores have not been honed to death and the gun still has .016-.018 wall thickness it might have always been “below wall thickness”. Wall thickness has nothing to do with proof. I was told that a gun recently passed proof with one barrel of .015”. In proof, is in proof, but you still need to evaluate the gun for what it is. I doubt that there are many 2” guns with barrels thicker than .020”.
