In 1860, Forsyth noted his preference, or the correct way in his mind, was that they should shoot parallel - but - he noted in all of the many doubles he'd owned and tested, that only one shot perfectly parallel and such a treasure it was, with which he once shot a Sambar Stag at 250yards, the ball passing through both shoulders.

He had one he actually got quite vocal about that was regulated "so bad that it shot 6" apart at 100yards - he noted that if you ever got one that bad, to smash it or give it to a friend. (pick me)

Perhaps muzzleloading round ball DR's are more easily regulated than modern CTG. DR's.?