When looking at barrel convergence, I like about 15 yards.
It's easy to find the center of the pattern at that distance.
Contrary to the belief of some of our members the average flight path of a shot cloud is, in fact, a straight line as far as lateral convergence goes. Vertically, all firearms are compensated for gravitational drop and a shotgun is no exception.
Patterns that are coincident at 15 yards can only be off by the distance between the muzzles at twice that. If a gun crossfires at 15 yards, expect double the error at 30.
Vertically, just like a rifle bullet the center of your pattern will cross the sight line twice. Once on the way up, and once on the way down. Thus, the vertical POI must change with distance.
