I would think The trick to shooting groups for regulation would seem to be to shoot multiple(?5) rounds or more from each barrel at each distance and determine the POI of each barrel and if your load is spot on where you want it at various distances. If so then possibly game at 300 yards would be in danger should you ever did wish to try such a loong Double shot.

Shooting a smaller number, ?2 shots at each distance may not give a representative POI for each barrel.

Am I wrong or is there a better way to do this??