Point of impact (POI): most side by side and over and under shotgun pattern testing is done at 16 yards when testing the point of impact. Heavy duck guns and goose guns may be patterned at a longer distance. You need to fire a three shot group at the patterning board to get a good average of the POI, 3 shots at one sheet from the LEFT barrel and 3 shots at a different sheet from the RIGHT barrel. If the centers of the average of the 3 shot group are more than 3 inches different at the correct patterning distance then you have a bad problem, the barrels have not been properly regulated.

This LINK provides a very good description of how to do pattern testing.

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