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.
I am not addressing what physics are involved or the convergence rate of the barrels. Just "If the barrel centerlines are .85" apart and POIs converge at 40 yards, then halfway (20 yards) the POIs will be .425" apart. At 60 yeards they will also be .425" apart." I believe that is a fact of geometry.
I repeat, .425" at 20 yards and 60 yards is closer than I can hold the gun to or even see.
Best,
Mike
