What interthem is talking about is creating a false shoulder. If you size the whole neck up so that it wouldn't chamber and then set a NECK SIZE die to size only enough of the neck so that it will chamber with a crush fit, then you will be holding the case tight between the case head and the false shoulder.
A Lee Collet will not work since it is factory preset to size all of the neck (unless you had just the right size washer between the shell holder and bottom of the collet tube)
False shoulders are often used in cases like the Gibbs cases where you take new brass and set it up for fireforming. For instance, I form 30 Gibbs cases by taking 35 Whelen and setting a neck sizing die to size just enough of the neck to hold it tight in the chamber. Keeps the primers from backing out and the case from thinning too much on the initial firing.
