I've heard that they thought the constant hammering of the horses' hooves on the hard, hard, highway compacted and improved the iron. Because of the way horseshoe nails are used - hammered into place, bent over and nipped off - they needed to be soft and ductile. Used nails - stubs - were collected, welded into bars, and then welded into barrels. I read somewhere that a pair of barrels weighing a few pounds would need some 15 lbs of stubs. The barrels show plain twist, not the fancy two and three stripe damascus popular later on.
Based on the "feel" when I wiped out the barrels, I think that they are taper bored. British gunmakers were taper boring shotgun barrels long before choke boring was developed.
I'm assuming that the crown over IS is the barrel forgers mark.