The barrels are the Laminated Steel typical of higher grade guns in the 1870s
11b would be .751", but could have been honed to .762" and still be in proof.
Fractional bore was marked after 1887 and 11/1 would be .763".
0.150" wall thickness at 9" from the breech is the highest I've heard of or seen, so the weight of the gun is indeed remarkable.
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