I just spent an hour and three beers trying to come up with the precise reading.I could not come up with a reading that I would want to be the expert witness in a court of law.I did clamp the barrel to a table and put some Styrofoam behind the ball to keep it closer to the inside the barrel but I am not sure that worked any better than nothing.. I am going to try foam rubber next.I am sure of one thing RookHawk is right my W.C.Scott measured about .045 to .050 thickness 9 inches from the breech which makes me think it needs to at least be that thick 9 inches from the breech buy this was a 13 bore.

monty