How does the RCBS X-die work? Extend case life?

Discussion in 'Reloading' started by Fitch, Jan 23, 2010.

  1. Fitch

    Fitch Well-Known Member

    Messages:
    478
    Joined:
    Aug 5, 2009
    I started a new thread to avoid hijacking the one on case length.

    I always make sure my brass is under SAAMI max. I understand completely why and how having long brass can blow up a gun. So I measure it and trim as necessary. No problem.

    Unlike most of my other brass, my .30-06 brass (1953 Win M70) grows like corn on a warm summer night. I can barely go two firings with out trimming and I usually trim it every time taking off .005" to .009".

    I've adjusted the RCBS full length sizing die (not an X-die) to bump the shoulders only .002" near as I can measure.

    So when the RCBS X-Die was mentioned in the other thread I went looking for it, but before buying one I'd like to "know" it will extend case life which means I need to know how and why it works.

    So far what I've been able to find out is that it acts like a short chamber and compresses the brass back to length in some fashion. Try as I might (trust me on this, it's easy to get me to buy a new tool!) I can't get my head around why that would extend case life.

    As I understand it, brass grows mostly near the base at or just above where the web in the case head thins to wall thickness. There is a good illustration of that on Varmint Al's web site, and I see it when I check brass with my RCBS case checker which can give me a profile of case wall thickness. I bought it because I was getting suspicious of my .22-250 brass.

    My .22-250 brass, before I rechambered the rifle, grew, which resulted in measurable case wall thinning in the form of a trough, ring, around the case just above the base. When I saw it getting to be .002" deep I pitched the brass.

    Back to my questions:

    1) How does restraining the case neck axially with the equivelant of a short neck chamber in the die do anything about the thinning just above the web?

    2) If it doesn't do anything about the thinning above the web, how can it extend case life?

    3) Brass isn't a compressible fluid. The case has to act something like a balloon full of water - squeeze it here, it gets bigger there - it's always the same volume, just a different shape. If the case stretches near the web, where does the extra length (material) go if the brass isn't longer after it's sized?

    Thanks
    Fitch
     
  2. boomtube

    boomtube Well-Known Member

    Messages:
    1,595
    Joined:
    Oct 8, 2007
    "I've adjusted the RCBS full length sizing die (not an X-die) to bump the shoulders only .002" near as I can measure."

    Case stretching and eventual head seperation comes from the body wall just above the web, as you noted. The X die can do nothing, of itself, to control that.

    The X die works it's magic with a decapper stem that has a dual diameter rod with a square shoulder transition. The larger diameter upper part is adjusted with the step making firm contact against the case mouth and serves to push any lengthening back into the case shoulder. That obviously eliminates the need for trimming but it does nothing to stop case wall stretching or extend case life.

    I believe you need to reduce the shoulder bump. I adjust my FL sizers, including .22-250 and -06, to duplicate the fired and resized shoulder location and my cases last a long time. A case is expanded and stretched when fired but then springs back for easy extraction; that includes the shoulder location. By setting your's back even further each cycle you only insure addtitonal stretching and it likely has little effect on chambering effort.
     
    Last edited: Jan 23, 2010
  3. Fitch

    Fitch Well-Known Member

    Messages:
    478
    Joined:
    Aug 5, 2009
    Suspicions confirmed. Thanks.

    Your suggestion makes sense. I'll try backing off on the shoulder bump even more to duplicate as fired measurement and see what happens. It won't take much, I'm only bumping it .002" now near as I can measure it.

    Ohdarn. I need to head for the range to make some fired brass so I can re-adjust the die. It's tough but somebody has to do it. :D

    Thanks
    Fitch