Wall thickness can depend on the gunmaker ,the the age, the material its made of and the use its going to get. Although there are several theories as to what is an ideal,or what constututes a safe margin will also depend on these points.
It was accepted in the Birminghan that a Greener for example needed to have heavier walls than the equivolent Webley.This was down to the simple fact that Webley used a "higher" spec steel.
There is no hard and fast rule as to the maximum or minimum wall thickness "recommended " wall thickness was invented by the auction houses as a way of selling worn out guns with no come back ,I belive this has now stopped and they just publish a list of bore sizes so it down to the buyers discresion. Although most will not sell agun at under .015"other than as a stock and action.
