When trying to find a load that "regulates" in a double rifle, often the problems come from "bench techniques" rather than the ammo.The rifle should be held in the hands, The front hand can rest on the bag,but never rest the rifle barrels.Always shoot both barrels, the second, just as quickly as you can reaquire the target. This makes it hard to know which hole came from which shot.To account for this, I set two targets and shoot the right barrel at the right target and the left at the left target.Later, one target can be placed on the other and location of holes marked on one of them.I always let the barrels cool to ambient temp.This does take more time, but I usually have something else to shoot on another target.This makes it easy to see if the barrels are crossing or shooting "wide".You don't have to shoot from a "standing rest", but you should stack the bags high enough that you sit upright and "rock back" at the shot.You can use a rear bag, but it should not impede the movement of the body.Usually three shots from each barrel is enough for each group.The idea is to get the barrels to shoot to the same POI. The location of the group can be moved by the sights.The rifle should have been regulated at 80-100 meters, if you know the distance,use it for your shooting.If you don't know, I suggest 100 meters.If the rifle has a scope, it must be used in finding the load.The rifle may not shoot to the same place with the scope as without. If you mount a scope later, you may have to have the rifle regulated again.The basic "rules" stated above are correct.
Mike