Dean's answer would be the short version
and it's a lot more complicated.
Since I've never found engineering drawings or specifications by ANY U.S. maker for wall thickness, the recommendations are based on lots of measurements of presumed to be unaltered barrels.
http://parkerguns.org/pages/faq/BarrelThickness.htm
Never missing an opportunity to rant
, sellers commonly list MWT, never bothering, or not disclosing, end-of-chamber and 9" numbers.
Consider this
