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shoulder bump in .223 AR

Discussion in 'Handloading and Reloading' started by fishshocker, Feb 13, 2013.

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  1. fishshocker

    fishshocker Member

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    Gun: 20" Wilson barrel with Wylde chamber RRA Ar-15.
    Load: 69 smk with 25.3 Varget at 2.260 oal. Shoots 0.8-1.2 moa at 100yds, but about 2-2.5 moa at 300 yds.

    I've been using a FL Lee die set up the way the instructions say to, screwed down until it touches shell holder and then another 1/4 or so turn in.

    I have not bought a case guage since I figured 1)my reloads chamber and run fine, and 2) I'm sizing them the way Lee says.

    I just got to wondering about how much shoulder set back that Lee die does. I was fiddling around and took a fired and trimmed square .270 case and put it on my calipers like a stoney point headspace guage and then put one of my fired cases in it (the .270 case mouth rests about 3/4 of the way down the .223 shoulder). I measured several fired cases and all gave the same measurement.

    I then took some of my resized brass and measured the same way. All pieces were .009 shorter, aka .009 shoulder bump. Most people say only bump back .003 in a AR.

    My questions are:
    1) is this brass still usable bumped back .009"
    2) how much does excessive shoulder bump like that affect accuracy.
    3) is using a .270 case like a headspace guage caliper attachment alright to do. (I was getting repeatable measurements with it, just had to be very careful to keep each case head centered on caliper jaw and squared up.)

    I've since readjusted the die to give me .003" set back and that brass chambers fine, but I have not loaded and shot any to check accuracy gains.

    Thanks,
     
  2. rcmodel

    rcmodel Member in memoriam

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    .009" excess headspace raises the probability of a case separation when fired considerably..

    If you reload for an AR, or any other semi-auto, do yourself a favor and buy a Wilson case gage. They are very cheap insurance.

    And will give you enough additional case life to pay for themselves in short order.

    rc
     
  3. fishshocker

    fishshocker Member

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    here's a pic showing how I was using a .270 case as headspace guage.
     

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  4. rcmodel

    rcmodel Member in memoriam

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    I understand that.

    But your method does not measure from the actual datum line on the shoulder, so you have no way of knowing if the headspace set-back is within SAAMI specs or not.

    Thats what the case gage does for you.

    rc
     
  5. Walkalong

    Walkalong Moderator

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    The case gauge is cheap insurance for sizing cases for a semi auto..

    [​IMG]
     
  6. 243winxb

    243winxb Member

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    Fishshocker, mine is made with a 223 case. Works well, better than my case gage. If i subtract the length of the gage (case) the measurement is within .003" of SAAMI. Brass will separate at .014" shoulder set back from fired brass. Any amount over .010" is not good. Best at .005" IMO. [​IMG][/URL][/IMG] [​IMG][/URL][/IMG] Click photo for larger.
     
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