Further to Dig's comments: partly due to the new stringent application of the CIP rules, a much larger proportion of guns are failing proof, including new guns by some of the best names.
The particular problem is bulging at the choke's forcing cone and different choke profiles have been adapted by several Makers.
In the past, a gun failing reproof was a rare and noteworthy occurrence, nowadays I expect it to happen to about one in three.
They have all gone through eventually but various remedial measures have to be taken to get them through the 2nd (or 3rd) time!
And I am not talking about guns with super thin barrels; some have been sleeved guns with generous wall thicknesses.
Also there is no prevalence of damascus nor steel: they suffer equally from the rigorously applied CIP rules.
