Alan

I think I see what you are getting at. The answer is I believe I read it in either Greeners Book or maybe in the Shooting Field. Or perhaps Burrards book.

Perhaps the better answer would be the parallel distance would be the distance between the barrels at the point the bullet leaves the barrels.

This would not make the Double barrel groupings that accurate vis a vis both barrel groupings. It would be interesting to see how a perfectly regulated rifle actually behaved at 300 yards or longer. Unfortunately, I have never seen or even heard of such a creature.