The one I had was much shorter than the one pictured here. It did not have forward contact with the bore at all, fit only to the chamber. It used three point contact at the forward end of the chamber to sight in. two were slides, the third screw adjustable. This third one was loosened to remove & when re-inserting it was tightened to a preset stop which was set upon sighting in. As long as I had it it always came back to zero upon removing & replacing.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra