Monty, after many years of experimentation and use of the Manson gauge in all positions and with and without the Gaddy modification, I now only use the gauge in the horizontal position with the barrels held in a vise or by a helper. The other poster is right about clamping the gauge in a vise. It doesn't work. You clamp the barrels, support the gauge end with a short rope loop and zero the gauge while holding the the ball between the thumb and index finger of the other hand. Then insert the ball in the barrel and take your readings. Rotate the barrels for readings on 90 degree surfaces and on the other barrel. This method was taught to me by the master, Mr. Jon Hosford. Sorry for the edit. I think I got it right this time.