Trim the case once.never trim again! The X-Sizer Die looks like a regular Full Length Sizer Die. Inside, the die features a specially designed mandrel with an expander ball and decapping pin attached. The mandrel is what the X-Die is all about. The patented X-Sizer Die eliminates the need for repeated trimming after an initial trim of .020" off the maximum case length to standardize the case. The X-Die doesn't shorten the length of the case: the mandrel reduces the growth rate as the case grows as a result of the case mouth.contacting the mandrel during sizing. Because of an extremely close tolerance between the mandrel and die neck wall, the neck wall of the case does not thicken as the case length is pushed back. Cases repeatedly sized in the X-Die will initially grow a few thousandths of an inch, then stabilize below the maximum case length with no discernable loss of accuracy or case life.