SmallCaliberCentral

I tried to search X Die but the X was always ignored.

Anyone using the X-Die? Whats the good and bad about it?

Never tried one.
The X die does its job by pushing back the lips of the case when they reach a specific length.

I've never understood why RCBS would want to do this. I know I would not want a die pushing back on the fragile lips of my brass. Especially in a long throated 204 chamber where consistent neck tension plays such a big role in accuracy.
Theoretically I can envision a X die putting a slight crimp on overyly long brass before ever seating the bullet.
Not a good thing. The Xpander ball should iron it back out but now your working the necks more than needed.
I can only wonder what kind of run out problems this might cause.

Excessive case stretch is caused first by weakening the web while fireforming short headspace brass in a longish headspaced chamber. This creates a weak spot in the brass that will continue to stretch more easily.
Quality brass is usually headspaced more closely to saami specs and needs to stretch less during the initial fireforming.
For instance
Nosler brass headspace measures .003 less than my go gauge.
Winchester brass measures .010 less.
In my Savages I have the ability to reset the headspace according to whichever brass I intend to use. Even in my Savage I could not reduce the headspace as much as I would have liked to fireform Win brass. The bolt head would bottom out on the barrel.

Improperly set up FL dies are another big culprit. Dies need to be set up to bump the shoulders back just enough to ensure proper chambering, no more.
The die instructions that simply say screw it in 1/4 turn past ram contact are wrong.

Since I put down the go guauge and began setting my headspace for a crush fit(if possible) on virgin brass my trimmer has had a pretty lonely existence.

If your shooting a semi this info is probably not valid and case stretch is a way of life.