I did find this interesting reading especially in the way things have changed when I was purchasing guns here in the past. Especially if you did what I did was to purchase guns at local auctions with the best guns always being a farm auctions, these nearly always had the lowest prices though with the largest risk of coming away with a bad purchase. It was always caveat Emptor with no returns. Now a lot of guns where as you could say especially polished up for the event with barrels well reamed and polished for the occasion so you had to be able to spot them easily. My main interest was 12 bore because this was the most popular gauge here because smaller bores the cartridge where more expensive and not popular with the only exception was the .410. Now I am sure you all know that a tenth of an inch on this side of the pond can turn a gun from desireable to junk instantly. So to put the odds in my favour I would have two Gauge's that I built on the kitchen table one was a go no go bore Gauge with two interchangeable inserts .729 inch the standard 12 bore barrel diameter and the other .740 inch the end of the line for a 12 bore barrel having a 9 inch stop. The other Gauge was a short light pocket version of a standard barrel wall thickness gauge that was also set to 9 inches, this 9 inch distance from the breech is where the guns bore is measured also the wall thickness and to be sure measure the barrel thickness 9 inches from the muzzle. So if the .740 inch went into to the bore 9 inches though I have had it go in the bore to the handle the gun was out of proof though all was not lost if the barrel wall thickness was over thirty thousands of an inch there was a good chance of re proof but only if there was enough thickness at 9 inches from the muzzle say twenty five thousands. Though you did have to use your judgment of how much metal would be left after all the pitting if any was removed, so the gun did have to be a top maker in good condition to take a risk. The other rod for the wall thickness gauge was for .410 though hardly ever used the two Tdi's one Metric and the other Imperial. Though now all this is now done for you because last time I went to a sales show there where Gauges on every sellers table

[Linked Image from i.imgur.com]