After finally getting my Bill Henry wall thickness gauge, I set out to measure every vintage double I could find, starting with my lowly Smith 00. To my dismay found the chambers had been lengthened to 3 1/4" along with lengthening the forcing cones.
Fortunately, 1906 Armor steel 00 barrels are stout, and MWT as follows:
Right: Chamber @ 2 1/2" .132", 3" .118 and remained .118 out to 4", 6" .088, 9" .053.
Left: Chamber @ 2 1/2" .125", 3" .112, 3 1/2" & 4" .111, 6" .080 and 9" .044.
I very much agree that this outcome certainly may NOT occur in a light weight or small gauge barrel, as per the images posted at the beginning of this thread.

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