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RCBS X dies.

13434 Views 9 Replies 8 Participants Last post by  zb338
I have a 7MM Remington Mag. RCBS X die. When I
tell my shooting buddies that I own a die that sets
it up so that you trim your brass one time and never
have to trim again. My friends don't believe me. Does
anyone out there understand the workings of an X die?
I would like to understand how it works and be able to
explain it to my shooting buddies.

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Not sure how it works, but they do, and I love them.
I can't explain it either,but I know that they work as advertized in my 30/30 Winchester.
This die is not intended to shorten the length of
the case, rather, as the case grows, the mandrel will reduce the
normal rate of growth. This is the result of the case mouth coming
in contact with the mandrel stop during resizing. Cases repeatedly
sized in the X-Sizer die will grow a few thousandths of an inch initially,
and will then stabilize below the maximum case length.
Using the X-Sizer die eliminates the need to trim after each shot.
X-Sizer dies can be used for full-length resizing with or without
the X feature. Partial neck sizing can be done but only without
the X feature.
I'll believe this when every company manufacturing reloading dies markets the same type die setup. Otherwise its only so much Madison Avenue advertising hype in my book. If it worked so well it would sell itself instantly but instead RCBS is not pushing the marketing. This all makes me wonder how well it really does what everyone says it does. The review I read after there introduction indicated it does not eliminate the need for case trimming only reduces the frequency trimming is needed.
Nosler has advertised using their brass and the X Die to go 42 loadings on .223 without needing to trim.
I'm not saying the die actually WORKS, but the metallurgy doesn't seem all that complicated. The one question I would have goes something like this:

If the chamber in YOUR rifle is considerably longer than the original trim-to length specified by the die, and the loads you shoot are pretty stout, that brass is going to flow forward, so what does this "mandrel" do with that additional brass in the neck?

If it is able to automagically press the entire case length back, and not just the mouth/neck of the case, then I can see how trimming would be all-but eliminated. If they haven't figured out how to perform that minor miracle, I cannot begin to understand how they keep the case length from growing without resulting in a thickening of the case neck.
In my experience, resizing causes more brass flow than firing does. It's interesting that this is not a new topic; was just perusing a 1967 edition of the American Rifleman that points out how "partial full length resizing" can make the case shoulder move forward to where a fired case won't chamber anymore. So nothing new under the sun.

Expander balls that have to be pulled back through the case mouth are another culprit, again in my experience, unless ultra-smooth (carbide) or lubed.

As far as why everyone doesn't use the same dies... well there are production costs associated with design changes, and then, there's patent infringement (assuming RCBS patented their design).

Lotsa choices for the reloader, that can be a good thing.

Mike has it correct. The X-die has issued patent #5,635,661. It was filed March 13, 1996, so it will expire March 13, 2016. After that, you can expect to see imitations to appear.


This die works by using a step, or shoulder, in the mandrel to push the stretched neck brass back down into the shoulder. The X-die instructions note that overdoing this will cause a buckling of the case or a bulge in the shoulder. That is why you trim extra short just once and let the case settle into use. It minimizes the push back.

The die design does two things that appear to be advantageous: One is that it relieves you of trimming, except you will still need to add a chamfer if you use one, unless it is built into the profile of that mandrel shoulder. I can't tell from the illustration and I don't yet own one to check? If not, that step will iron your chamfer out. The other is that, like a Lee Collet Die, the X-die mandrel appears to need no expander to keep the ID of the mouth correct. This avoids pulling by an expander, which is famous as a cause of off-axis necks that make for runout.

The only thing I don't understand about the die is how it compensates for different neck wall thicknesses? The reason standard dies have an expander is they first over-resize most necks by having the neck portion diameter small enough to size the thinnest neck in the SAAMI spec. Then the expander corrects the ID if a thick neck went in and got squeezed down to too small an ID (the usual case)? The X-die design, from their instruction illustration, appears to be a setup such that if the mandrel is the correct I.D., the neck sizing portion would have to be sized to accept the thickest necks, or they would jam. If so, thin necks would not be sized down enough to touch the mandrel until they had grown long enough for the mandrel shoulder to upset the mouth fatter? So I don't see how that is working?


Have you measured neck runout on the cases coming out of your X-die? Is it near zero, like the Lee Collet Die produces?
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Thanks for your input Gentlemen. Uncle Nick
seems to have a handle on it.

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