The left barrel was made to be used with a single projectile (bullet). If it had been made after proof laws were implemented, the left barrel would have been proofed as a rifle. Early on, brass cases were available in bore sizes from 40 to 76 mm long and even longer. The right barrel was made to shoot shot; the straight grooves were thought (hoped) to give room for black powder fouling. I have recently read the purpose was to concentrate the shot during "pre choke" days, I can't say what they were thinking, but the utility for this is doubtful.
Mike
I have a Sauer, steel barrels, that is somewhat similar. It has a 16 gauge smooth barrel and a right barrel chambered for a 20 ga brass cartridge 40mm long. It appears to be a 16 gauge barrel like the left smoothbore, but it was bored to 20 ga, rifled to 16 ga and chambered to use a 20 ga brass case with a 16 ga ball seated. It functions well and is reasonably accurate. These were made in numerous lengths of cartridge.
I also have a French gun that is full length rifled in one barrel. No choke and very shallow rifling; only a few thousandths high and very slow twist. The smooth barrel is choked IC. These were marketed by the maker, I believe, as a Woodcock model. The idea was that the left barrel was the "tight" barrel and the right barrel acted as a spreader barrel for even closer shots.
I believe the statement is correct that it was intended to prevent spinning in an effort to keep the pattern tighter. Given that the right barrel is rifled to stabilize a round ball or slug, I suspect the intent was to shoot buckshot through the left barrel in as tight a pattern as possible.
