the reason is because it can effect the accuracy..and i agree shooting a shotgun with shot shouldnt matter..usually its shooting a rifle..a barrel is a harmonic device and i forget what that means but i think it has something to do with the vibrations and when the vibrations are disrubpted by say your barrel between the Y of a tree it can send the bullet off its mark...and if you have a bipod or a slung or anything that is attached by a band around the barrel im not sure what that would do to it..if the bipod is on the stock you should will be fine...