The British rules of proof make no reference to wall thickness of gun barrels. Barrel diameter and chamber dimensions are the controlling dimensions specified in the rules of proof. The minimum practical wall thickness recomended by the British trade so as to minimise risk of damage [dents] in the field is .020 inches. New guns often start life with minimum wall thickness in the range .025-.030 inches. During the past decade as the interest in shooting with Vintage guns has grown, so has the concern over wall thickness. If your gun is, "In Proof" it will be safe to use provided applicable working pressures are not exceeded. Wall thickness over .020 inches provide increased durability and longer potential barrel life.
A corespondent on this site recently reported that two Damascus barrel guns with .018 inch wall thicknes past Birmingham proof with out problem!
