I just examined a Boss SLE at Holt's with a minimum wall thickness of 14 thou. It had a visible rivvel (not a ring bulge but a distortion of the metal) which I could see clearly at the thin spot, probably where a dent had been raised - the proof house inspector must have missed it. The gun had 2011 proof marks on it. It should not have passed proof. It needed sleeving or re-barreling.
The proof house has no interest in wall thickness. If the material is strong enough to withstand proof charges it can be one thou thick, it will pass - if it is not strong enough at 45 thou thick it will fail.
You cannot guess wall thickness by looking at the gun. You have to measure the walls carefully. It is very difficult to blow up a well made gun forward of the chambers just because the walls are thin. However, it is not uncommon for them to develop bulges and rivvels and dents as a result of their thinness. This is a concern as to value.
The Boss in question sold for 7,000 plus commission. With walls measuring 24 thou or more it was a good enough example to command a total price of over 20,000.
Buying a gun with thin but functionally adequate walls is a means of having and shooting a top quality gun for low quality money. A client of mine keeps a Lang Imperial sidelock here. He shoots it every time he visits from N.Z. It cost 3,000 because it has one wall down to 18 thou. With thicker walls it would be a 10,000 gun.
He made the informed choice that rather than a very solid but ordinary boxlock ejector with thick barrels, he wanted a best quality sidelock ejector for the same money to use when he visited the UK for driven and walked up game shooting. It works, it was a good choice and he paid the right money for it because he was in possession of all the facts when he made he purchase.
There is no 'right answer' on wall thickness, there are informed choices to be made and to make them one needs to in possession of all the data and have the knowledge and understanding of it to make them.
