With British guns the barrel wall thickness is only one part of how healthy a gun barrel is. Now the much more reliable measurement is the bore size at nine inches from the breach face.
Take a 12 bore shotgun its bore should measure .729 inch at 9 inches from the breach face though the same gun will still be in proof until it reaches that dreadful stomach churning figure of .740 at that size it is living in the last chance saloon. Because any figure greater than .740 puts the gun now out of its proof limit and in the UK it is not legal to sell barter or gift, it can only be sold to a registered firearms dealer or given up to be scraped or to be brought up to the condition that it can be submitted for reproofing at the next bore size larger though you can still use the gun your self.
So knowing the two figures you can tell how much metal has been removed in back boring, removal of pitting, polishing, or dent removal, and therefore the guns state of barrel health. Though the figure of 25 thousands of an inch at 9 inches from the breach face wall thickness is seemingly the one to be recommended as the minimum wall thickness but I have known thinner.
