The British proof house prefers .025” at 9” from the breech end.
Mike, You have offered a very good and comprehensive list of what a buyer needs to know, thank you! However; your above statement I believe is inaccurate. The Birmingham Proof House measures the
bore diameter 9" from the breech by sliding a plug gauge into it, the size of the last plug gauge to reach 9" is then stamped on the barrel flats. They will inform the submitter if the MWT
anywhere in the barrel that can be measured (ribs are required to be fitted, therefore; the wall under the rib cannot be measured) falls below .020---even though it may have passed proof with a MWT less than .020. This information I got from a direct e-mail with the chief inspector, Michael Lynch, of the Birmingham Proof House.
