The experience I have had with a rifle providing a shotgun pattern has always been scope failure. I read your friend scope is not on a picatinny rail, however, can you or will he let you pull it out of his rings and add it to yours? Something else I have wondered, how are you cleaning the barrel, do you have a bore scope, cannot remember what has been mentioned before? One other thing you may try is using Kroil and JB bore paste to clean any stubborn residue. My first thought during all this is your scope, I would eliminate it first, I would not worry about your Chrono results, shooting at 100 yards will not give you any usable data that will matter. I would focus on scope first and how you are setting up your shooting form.