I bought a Manson Precision Wall Thickness Gauge (I'm not coughing up the $$$ for a Hosford) and I'm getting results that vary too much to be right...so I'm doing something wrong.

[Linked Image from brownells.com]

I'm sure there's a proper technique, and I'm not doing it that way.

Obviously I have to keep the ball on the end in contact with the inside of the barrel while attempting to keep the arm it is attached to parallel with the barrel wall...but I must be moving it around too much.