OK, we have had wall thickness gauge discussions before. I hope you look at Drew's set up. The secret of using the horizontal tool is that it uses gravity [a constant] , and uses a string or similar to hold the out of barrel end to push the gauge in and out. This eliminates any torque you might put on the gauge if you just grab it with your hand to push it in and out. I was skeptical of the horizontal gauge, but used in Drew's way, it is 100% accurate. I have a vertical gauge, but even with the barrels suspended from a vertical spring, I cannot get readings as accurate as the horizontal gauge gives on "double barrels" My vertical gauge now sits on the floor of the furnace room.

http://www.bakercollectors.com/