The CIP figures are plain engineering figures pitting pressure against steel strength and the resulting figures are somewhat alarming. There is no way in the world I would trust a 12 gauge with 1.05 mm thickness at the chamber ends!!!
However, practically it is virtually impossible to work with those dimensions, firstly because the barrels would be impossibly light and whippy, secondly because the CIP figures do not include safety factors, gunmakers usually work to a factor of 2, so they would choose at least twice the thickness, and also because you need some spare metal for repairs and rebluing.
Even a dense designer must realise his creation will span several generations of users and will face repairs.
Having miked some of my doubles, it is reassuring to see that the thickness at the chamber/forcing cone junction is at least twice the recommended CIP thickness and in the 100 to 120 thou range in all except a Westley Richards droplock which is due for rebarreling anyway.
Other doubles I miked turned out to be in the 80 thou range for most Spanish and over 100 for most Italian, British and French.
Striking up barrels is a tough one. It gets easier with hollowed out backing blocks for the emery cloth, but it is never an easy job. Here you appreciate the monobloc method of barrel making that lets you do the maximum finishing on the tubes, using a lathe before they are put into the monobloc. In some modern lathes there are sensors that continuously monitor tube thickness and automatically adjust for the desired contour.
