There was a long discussion on this bbl convergence a while back which ran into many pages. I believe that it was finally & generally conceded by all that the bores on a SxS double do converge from breech to muzzle in as straight a line as the makers could accomplish. This convergence was given in the Plans & Specs for the L C Smith FW as about .011" per inch of barrel length. An extension of the b static bore axis of the two barrels would thus converge about 6' from the muzzle & at 40 yds would print about 15" apart, the axis of the right bbl being to the left & that of the left bbl to the right.
That one would not expect the two bbls of a well made SxS to print its patterns 15" apart at 40 yards is I believe conclusive evidence the barrels do not remain static during the shot's travel down the bore. The spacing of the two bores of a 12ga would be expected to be around .800" depending upon actual bore size & actual wall thickness. Bores of .730" with wall thicknesses of .035" assuming they touch at the muzzle would result in this .800" spacing. I can't recall that I have ever seen a 12ga ouble with the bbl centers being anywhere close to 1.00" at the muzzle.
