Gene,
French proof, by law, was always the highest in Europe. While I'd always advise an accurate measurement by a GOOD gunsmith (not Bubba, the 870 mechanic down the street) it would be an unusual French gun with lengthened chambers that would be un-usable with ammunition that was a least in the vicinity of the pressure it was originaly designed to use.
Anecdotal story: When I was in St. Etienne, I had an R10 restocked to fit me. Someone stateside had decided it needed it's chokes relieved, and the gun had suffered something that left pimple bulges in the left tube. Some genius also decided to backbore the tubes. These are heavy barrels, and the thinnest area the maker could find was still .060, and most, was much closer to .090.
They assured me it was safe to use as is. I have used it, even with the Federal "Pheasants Forever" loading of 1 1/4oz of fives for late season roosters here in MN. I have guns that typically get the nod over the Darne for that work these days, mostly because the Darne has no and little choke left in the tubes, but, I wouldn't think twice about it, if the situation called for it.
No problems to report, save the little guy does let you know, in no uncertain terms, when one of those rounds has gone off in the gun. I don't notice recoil when shooting in cold blood, however.
Measure, first. But, French guns, especially, will usually be OK.
Best,
Ted
