This is a Sinclair concentricity gauge that I have modified so I can measure runout at the bullet relative to the shoulder. The modification so I can get large cartridges turning on the center of the shoulder taper.
In the case of the test on dies, I was using 223, so an unmodified gauge would work.
The firing pin pushes the cartridge forward until something stops it. I would like that to be the chamber shoulder. To keep the bullet concentric with the bore, I like the bullet touching the lands to center the front.
To center the rear of the bullet, I want the inside of the case neck to be concentric and parallel with the chamber shoulder. To measure that I want to turn the cartridge in the gauge on that shoulder and put the dial indicator stylus on the bullet as close to the neck as I can.
Redding FL "S" dies do poorly on that test.