Concur with KY Jon - this gun appears too nice to monkey around with lengthening the chambers and 2 1/2 inch shells are readily available (or makeable, if you reload). French hammerguns are pretty thin on the ground and this one looks exquisite. The big question, though, is: What do the inside of the bores look like. My experience is that they come pitted a lot of the time and you have to be darned careful about that. There's not a lot of meat in these barrels and pitting (or reaming out chambers) can get a bit dubious.
The marks you write about:
"acier decarbure" - decarboned steel
"17.0" - the bore diameter in millimeters, which translates to a 16 gauge
"6.5" - the chamber length in centimeters, which translates to 2 1/2 inches
"St. Etienne" - the proofhouse in St. Etienne, France, where the gun went through proof testing.
the wreaths - the number of wreaths indicates the intensity of the proof testing.
the crown with PW - an initial proof
"Eprouve Poudre Pyroxlee" - proofed with smokeless (nitro) powder
