My understanding of regulation for S/S DR's is that the bores cannot be set up parallel to each other, since recoil causes the right hand barrel to shoot to the right of where it is bore sighted and the left hand barrel to the left. For this reason, the barrels have to be set so that the ballistic curves converge at a specific point, which means that beyond that point they begin to diverge. If the barrels shoot to the same point at 100 yards, then the right hand barrel will be shooting to the left and the left hand barrel to the right beyond that point.

Since the line of recoil of O/U barrels is the same as the bore line of both barrels, this does not hold true of them. Some hold that since game animals are larger horizontally than vertically, this divergence is of no consequence, but it seems to me that it is.