I can't say that I can remember ever reading of a "standard" distance at which barrels were supposed to be regulated, on a shotgun. When I test double guns for regulation I like to do it at 16 to 20 yards. I want a small pattern on the plate because it is easier to determine the center of the pattern if it is small. I have found that if the gun in question is regulated at that distance I don't have to worry about it at any distance I will be shooting game or targets.

Just my findings only, SRH

May God bless America and those who defend her.