I found Bill's protocol from 2012, with additional discussion
http://parkerguns.org/forums/showthread.php?t=7889&page=5
1. My gauge has a 5/16" hole in the "block", so I installed an eye bolt in the hole with the eye facing out.
2. I tied a generous loop of clothesline rope to the eye bolt with loops and knots to make holding on easier.
3. You need an assistant if measuring at a gun show or auction. The assistant needs no skill or strength.
4. The assistant will hold the barrels steady on a table with a cushion and the end to be measured over the edge. An assistant is to be preferred over a vise or weights, especially at a show or auction. No weight is heavy enough to hold the barrels steady.
5. The micrometer is zeroed with the rope holding one end horizontally and the ball supported at the other end by the fingers.
6. With the micrometer properly zeroed, the ball end is inserted into the barrel, run from the open end of the barrel to the extent of the rod and the results recorded. The rod must not contact the interior barrel wall for accurate results.
7. The assistant will rotate the barrels on the pad 90 degrees and the results recorded as in the previous instructions.
8. Another 90 degree rotation will complete the measurement of that end of that barrel. The fourth quadrant cannot be measured.
9. Do the same to the other barrel.
10. Reverse the direction of the barrels and do the same from the other end. This should result in 12 recorded minimum thickness measurements from one set of barrels.

I put 25# in the box in which I carry the gauge, and use bungie cords to affix the barrel. I can rotate the barrel while watching the gauge, but do need an assistant to record the results

[Linked Image from photos.smugmug.com]

And thank you dblgnfix for this post, which William apparently missed.
I make my own wall thickness gauge similar to the one Drew has in his post. I have verified its reading via other methods such as cross sectioning ultra sonic and eddy current. They all give the same reading within .0006" which wouldn't make the slightest difference.

And no, this thread is not about you William.