Mike;
This would be true if the bbls were static as the shot traveled down the bbls. That however is not the case. An extension of the bore C/L's do not converge @ 40yds, but much, much closer to the muzzles than that. Each bbl thus has to "Whip Out" for them to perfectly center at 40 yds. The spread @ 20 yds would thus be greater than indicated by just the muzzle seperation of the two bbls. It is though still nothing to get up on one's tricycle about & of no practical consequence.
As I recall without getting my L C Smith Plans & Specifications book out which gives bbl convergence for their guns at 40 yds the bore axises will have crossed & be spread by about 15-16 inches. The axis of the two bores converge at a rate of around .011" per inch. When I first acquired this book I checked the seperation of some of my Lefevers at both breech & muzzle & for this point they could have been built to the Smith specs.
