With the talk about headspace, makes me want to share one of my tools. Adjustable magnum headspace gages.
This one is for my 240 Weatherby, the flat head screw is held tight by a set screw, that is tightened from the other end. Ground to length, it can easily be measured for length.
My 240 is a straight wall, and I set the headspace short at .215, so I could turn the belts to .213 and only have .002 gap. Works well.