Rather...
When you run you cases into the Sinclair expander, do you run them in all the way until they bottom out and the case mouth bumps the shoulder on the expander mandrel? If so, this is likely the source of the bump back.
I only run my cases over the mandrel until it goes all the way into the neck, and don't let it bottom out.
Even then I will see the brass come of the expander a thousandth or so shorter than before the pass, but never anything like you describe.
And rather than induce any runout, I sometimes see the sized brass have a little LESS runout after the pass over the expander mandrel.
Hope any of this helps.
Rex
