Bob/Sieger's comment about the rim got me thinking, and I ran my 40 S&W cases on the lathe and turned the rim down to approx .042"...(from approx .052")...But when I ran my mixed bag of other brands/calibers, there was some variation, as much as +/-.004"...
I was using a live center and locating off the edge of the primer pocket...But they weren't all the same...Some had a tapered edge, some not so much...So my 'zero' wasn't quite right...
So to make a better 'zero' I turned a piece of Nylon down to fit in my tailstock Jacobs chuck with a small primer 'nub' [.175"] on one end and a large primer 'nub' [.210"] on the other end...
Now I clamp this mandrel in the Jacobs chuck, set it at zero, push a case up onto the mandrel and then clamp the neck in the lathe chuck...Case is centered and rim base is up against the mandrel...And now I can cut my rim...
Works much better! Variation is cut down to approx +/-.001"...
I would rather use Teflon for my mandrel...But I'm all out...
