Jewler and Pooch,
Have faith! I too purchased a Manson wall thickness gauge from Brownells and mounted it to a wall and immediately figured out that there was no way to get accurate, repeatable readings and thought gee, that was a waste of $100.00!
After much research, I have discovered that the way to use it is exactly as Eightbore described. Use it just like the Hosford gauge.
Watch the Hosford video to get an idea about what we're talking about. Then thread a piece of rope or webbing through the hole in the centre of the gauge. Zero the unit by holding the rope (so you do not influence the reading)and the sides of the ball. Slide it into the barrels and read away! Make sure that the ball is the only part touching the inside of the bore otherwise you will get a false reading. It is easy enough to get the hang of provided you can keep the barrels in place. I suggest putting the barrels on a bench and anchor them down with a couple of bags of shot. Pre load the plunger with .200" (as I recall) as per the instructions.
I can get accurate, repeatable readings with this method and I think it is useless if fixed in a vertical position.
Gravity does the work of keeping the ball in contact with the inside of the bores and the plunger is not affected as it is on the opposite side.
