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300 WSM Headspace question

Discussion in 'The Ammo & Reloading Forum' started by lobo55, Oct 11, 2009.

  1. lobo55

    lobo55 New Member

    Joined:
    Oct 11, 2009
    Messages:
    2
    I have a Kimber 300 WSM that I have been reloading for. I have a hornady headspace guage and the RCBS precision mic. The measurements are:
    Hornady headspace guage measurements:
    Comparator + 0.420" bushing + Fireformed 300 WSM case= 3.741"
    Comparator + 0.420" bushing = 2.003"
    Headspace = 1.738"
    RCBS Precision Mic measurements:
    1. Fireformed 300 WSM case= 1.7260"
    2. Fireformed 300 WSM case= 1.7260"
    3. Fireformed 300 WSM case= 1.7258"
    4. Fireformed 300 WSM case= 1.7258"
    5. Fireformed 300 WSM case= 1.7260"

    Average headspace using RCBS precision mic= 1.7259"

    RCBS precision mic table shows minimum dimension-1.7260" maximum dimension-1.7360"
    so there is a 0.012" difference using 2 different methods. I know as long as i stay consistent with measuring and setting my resizing die there will be no problem but my question is-there any reason anyone can think of why there is such a big difference?

    Thanks-lobo55
    Last edited: Oct 11, 2009
  2. Popgunner

    Popgunner New Member

    Joined:
    Dec 3, 2005
    Messages:
    1,764
    The RCBS precicion mic is made just for your 300wsm & has the shoulder angle just the same as your cartridges & I would guess it would have a more perfect datum spot. I would trust the precision mic to give your true headspace more than the Hornady.
    The Hornady is a good unit & can be trusted to give good "relative" results. Hornady's 5 die set is advertised to cover 17 calibers. Problem is the shoulder angles can be different & the datum point changes. I wouldn't try for an exact reading with the Hornady, you just use it in a relative way to adjust your sizing die to set the shoulder where you want it. Works great for that.
  3. lobo55

    lobo55 New Member

    Joined:
    Oct 11, 2009
    Messages:
    2
    Thanks Popgunner.....
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