I have a side lever opener W Richards with Birmingham proofs of the same era. This gun has a 12ga chamber with the bbl marked as a 14 (.693"). As on this gun there is no chamber mark, just the provisional, definitive & view marks along with the 14. The bores on it now measure very close to a 13 (.710"). At this point in time (pre 1887 as I recall) the bore was only marked with whole sizes, no intermediates (ie 12/1 etc) being used. This bbl could have been any size which accepted the 12 (.729") plug but not an 11 (.751") one at proof. It may quite well have originated having a bore just under an 11 with a 10ga chamber & been subsequently polished/honed out to the .758" size. May well be cylinder bore, mine is.
The construction of this bbl would have been alternating layers welded, then rolled down to size leaving the layers quite thin & then wrapped around a mandrel with the layers on Edge. There would have been no twisting in the rod prior to winding (Twisting) around the mandrel. This was often called Wire Twist, but each piece which has the appearence of a wire on the outside, actually goes entirely through to the bore. If you can imagine, take a piece of laminated shim stock & cut off a strip of it. Place it across a rod with the edge showing the laminations against the rod & the outside, then procede to spiral wrap it around the rod. Each lamination would thus be visable to the outside & go through to the inside. For damascus one end of that strip would have been fastened, the other end twisted round till it looked like a threaded rod, then hammered back flat & then wound around the mandrel. You then have the so-called "Finger print" pattern rather than a plain spiral appearence. For 2-strip, 3-strip etc damascus more than one of these twisted strips would have been welded together edge wise prior to winding.
