It is quite simple, the barrel is supported on 2- 3/8" ball bearings on a 5/8" dia. rod. The rod is setting in a V on each pedestal, the barrel is placed on the rod and the breech end is pushed up against the stop. The dial is then moved from the set position to anywhere along the barrel you want to measure.
There are only 2 points along the whole length of the barrel where it is in contact with the ball bearings, so there is only about .001 sag. That is the reason for the other ball bearing all the way to the right, it is there to set zero and to see if there is any sag.
The two points are 4 3/4" and 24" along the barrel's length.


David