Left,
You will need to set the shoulder back to get proper headspacing. A good way to get everything right is to measure the barrel to breech face gap with a feeler gauge.
Once that is determined, use a fired cartridge and measure the amount of case head that is sticking out the end of the barrel with a dial indicator. My rifle has a gap of .004", and once fired brass were protruding .005-.010" due to slop in the hinge pin and frame stretch.
Setting the shoulder back with a regular resizing die will work, but you can get some stretching when you pull the expanding ball back through the neck. What I did was to purchase a body die made by Redding. It will set back the shoulder without touching the neck.
I neck size my brass first, then I set the body die to set back the shoulder until I get .003"-.0035" of the case protrusion. This gives me a headspace of .001" or less. My gun now closes perfectly everytime, and my accuracy has improved tremendously....
Check out bellmtcs.com for more info on headspace issues.
I hope this helps!