When you figure the mathimatical convergence it will work out more than the actual convergence because the firing the right barrel will move the barrels slightly to the right and the left slightly to the left negating the mechanical convergance putting one pattern right on top of the other if the shotgun was build around an average load. Unless the barrels were very crudely built I suspect that patterns will be nearly on top of each other at least to the effective range of the load.

After the first shot the rest are just noise.