The book L C Smith Plans & Specifications gives the convergence of 12 & 20 ga FW bbls.
They are listed as follows;
12ga
Bbls comverge at the rate of .0113" per inch or an angle of 038'.
For a 30 bbl having a 1.156" spacing of C/L's at breech & .8175" at muzzle the axises of the bores would converge at 72.452" from the muzzles. At 40 yard the would be separated by 15.453". These Axis lines would of course cross after passing the convergence point.
For the 20ga with 28" bbl the convergence is given as .011" per inch, a convergence point @ 63.09" from muzzle & seperation of 15.146" @ 40yds.
Understand this is simply where the bbls are "Pointed" this does not mean this is where they are going to hit when fired.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra