There is a ball bearing that is in the shaft opposite the spring loaded arm. Zero is set each time and then rechecked.

The barrel whether I am measuring the muzzle end or breech end, fit into a tapered cone to center the barrel.
I mainly do L.C. Smiths and had to make the cone longer for measuring the breech end so that the extension rib did not hit the cone when rotating barrel to get readings.

David