Under the British rules of proof, the gauge is determined by a bore diameter measurement taken 9 inches from the chamber end of the barrels. All measurement to see if it is still in proof are taken at that point so I think that is what they call the "point of proof". I called Graham Greener with a question on a shotgun and asked him what he felt was the minimum safe wall thickness for damascus barrels and he said 25 thou., others might say 20 thou. Everyone has to decide on their own number for minimum wall thickness. I think I read somewhere the Birmingham proof house would often refuse damascus barrels submitted for reproof when the minimum wall thickness was less than 20 thou,. but that would partly depend on what the original proof was done at.
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