I've owned several Francottes, one an 1891 Grade 1 damascus. From what I've seen, Francotte 12 ga are chambered for 2-5/8" shells typically from between 1890 and about 1929. I have a 1929 gun with factory marked original 2-3/4" chambers. I had two gunsmiths check out my damascus barrels and then proceeded to shoot low pressure 7/8 and 1 ounce loads (nitro) with no problem. B&P makes a 7/8 ounce load which the factory rep told me was very low pressure ( the factory chart shows 6500). My gun had 30" tubes and still weighed only 6-3/4 pounds.
