For a British 12ga the following would be typical,Firing Pin centres 1 &1/8th apart (1.125") Muzzle diameters .800" with tubes touching.Thus centres would be .800" Total convergence rate of .011" - .012" per inch of barrel length. Bore Centre lines will intersect at 6-7' from muzzle. Upon discharge gun will undergo degree of torque rotation in direction of discharging bbl , ie Right pulls right, left pulls left. Impact will centre on line of sight along rib.
Actual pattern centre could not be identified accurately enough to express any convergence.
The gunmakers goal would be to have both patterns superposed over each other at 40 yds. The very theoretical incompleted convergence of 4/10ths of an inch at 20 yds would be impossible to measure
