Argo44,
You "discovered" a 7cm bore? What is a 7cm bore? Bore would have nothing to do with chamber length.
I have seen 76mm chambers on French guns. This is a 3" chamber, but, I'm, pretty sure that any gun proofed at this chamber length PRIOR to 1923 wouldn't have a higher level of proof just because of the chamber length. Level of proof was specified without regard to chamber length at that time.
I am also curious as to how you came up with such an exact date on a French gun?
Post 1964, French proof was the same for 2 3/4" and 3" Darne shotguns. Triple proof would have been applied to guns with either chamber length, and, it is more than adequate for either. One thing that did change was the steel used in the guns at the Darne factory, 2 3/4" guns got French forgings of XT, 3" guns got forgings of XTC, a higher chrome content tool steel. Either version is a tough steel.
I am relatively certain the American powder facts you have listed above, while very interesting, have little relevance when the subject is French shotguns of the same era. Damascus barrels were pretty much a thing of the past by about 1880 in France. It was the French who taught American industry how do produce steel tubes.
Since I neglected to do so in a previous post, I do want to take a moment to thank you for the time you took to put up all the different French gun making terms, and clear English definitions. That will be very helpful to all non French language speakers who are interested in French guns or gun literature.
Best,
Ted
