Another thing a lot of folks forget is the charge does not hit where the bbl was pointed prior to firing. The above mentioned L C Smith Plans & Specification book has a drawing showing the "CONVERGENCE" of the bbls in building. They further show the point at which a continuation of the two axis would cross & the amount those lines would be apart at 40 yds.
After acquiring this book when it firast came on the market some years back I checkede the doubles I had at that time, they all had very similiar convergence to the Smith Specs. I was of course working from the bbls ODs by measuring their dias & then across their outer sides.
Looking down the bores at a shadow line ( the method used by old time bbl makers for straightening bbls) did not show any of the them to have enough "Swamp" for me to detect. A pair of bbls which converged from the breech toward the muzzle & then turned parallel for the last 4" would be so easy to pick up a blind man could see it. No gun I owned fit that description.
I did not then own, & don't now, a .410 to check. It is quite possible that some of these .410s with wide a breech spacing & touching muzzles are swamped. I don't see how they could shoot to point of aim if not.
Do note though if you "Bore Sight" a double the two bbls won't sight to the same place, I "Guarantee". Their axises will cross not too very far from the muzzles.
