First I am trying to figure out what the question is. There are 3 paragraphs in your post and I really can't make head nor hair of what you are saying or asking.
However shotgun measurements and rifle measurements aren't related to each other. A shotgun is a point firearm while a rifle is a aiming firearm. In a fitted shotgun there are 2 reference points which your brain uses to point it at the target. The rear point being your cheek position on the comb of the stock and the front reference being the hand holding the forearm. (the bead on the end of the barrels really is not required at all to be a great wing shot) I rifle also uses 2 reference points, a rear sight and a front sight. They allow you to align the barrel so that it points at the target, whatever that is and compensates for bullet drop and windage.
I would suggest you go to a website for Track of the Wolf, they sell everything muzzleloader and have measured patterns.
