Forrest: The Birmingham Proof House does not reject barrels for proving based on wall thickness, and if that .018" is out toward the muzzle, it could well pass.
http://www.gunproof.com/Proofing/proofing.html
The barrels
are at much higher risk of a dent.
While we're here, some factory small gauge Parkers have been documented to have MWT in the distal barrels less than .020".
http://parkerguns.org/pages/faq/BarrelThickness.htm