Terry - "Nimrod" was a Francotte tradename. The barrels are Fluid Steel, likely Acier Cockerill of Cockerill Ougree-Liege.
Manufacture dArmes Feu Ligeoise used this steel almost exclusively, and labeled the steel Acier Universel. Some Browning patent Fabrique-Nationale-Herstal very early versions of the A5 shipped to the U.S. between 1903 and 1909 are marked Cockerill Steel. And Ithaca used Cockerill Steel for Flues No. 1 Special grade double for a period.
The gun was proved for a maximum 12g load of 1 1/4 oz. / 3 1/4 Dram Eq. (1220 fps) but the usual service (using) load was similar to today: 1 1/8 oz. 3 Dr. Eq. (1200 fps). The almost 110 year old wood would likely prefer 1 oz. target loads however.
Please have the gun checked and cleaned by a double gun specialist.
