Eight bore is correct. Nothing about the barrels orientation makes one gun more suitable than another for high long targets. Muzzle weight and stock shape yes. Clays are not birds but we shoot two high towers often never saw any hit advantage to a particular type of gun. Plenty of American SxS guns handle just like modern OU clays guns.

Reason you don't often see them in clays competition is fixed tight chokes. Most sporting courses are best shot with IC. Go long and high lots of Parker Smith Fox guns do very well

Boats