Here is a pictue showing how tension is put on the interior wall of the barrel. Opposite of that is the ball bearing. The rod for the 10 ga and 12 ga is 5/8"=.625 the ball bearing is 5/16"=.3125. The spring loaded brass bar is tapered and so is the slot. When compressed fully it has to be the same size as the shaft, 5/8" and the side that the bearing is on has to be drilled to less than half the diameter. As it is I cannot measure muzzle end barrel wall thickness on very tight choked 12 ga. guns, say .690 and less because of this.
The tapered brass cone that slides on the shaft and held with the thumb screw centers the barrel. With the breech end down it does the same.
Since the bore of the barrel is not concentric to the outside, you will always get a higher reading on one side than the other, but still you will get an honest wall thickness reading.
Here is a picture of the spring loaded brass and the dial needle resting on the ball bearing.
These gages are fairly easy to make, if I had a milling machine it would be a cinch to make these. The hardest thing is making the tapered slot and I did it on a 20" drill press with an XY table feeding .002-.003 at the most each pass. It still shook sometimes like a dog shi-ting razor blades.
