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Thread: Head Space

  1. #1
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    Head Space

    Hey all!! First time poster here. I am an experienced reloader, been doing it for 28 yrs. Rifle and hand gun. But this will be my first rodeo loading for a semi auto rifle. I just bought my 1st AR. What is the proper headspace on a .556? Yes I am sure it will safely shoot .556 ammo. It's a Spikes ST-15.

    I found on the 'net that max is 1.464, but the Winnie factory ammo I had measured 1.458.

    So what do I set my sizing die to?

    I know that I cannot "bump" the shoulder back .002" like I do for my bolt guns. .002" is not enough to reliably cycle the bolt (action) on a semi auto. Is this gonna be a "trial and error" exercise to find the proper headspace for "my" gun?

    This is the first question I have concerning AR's. Many more to come.

    Edit: I just measured my fired brass and they head spaced at 1.462". Reading this post again I think I answered my own question. Seems 1.458 is the magic number.
    Last edited by purple bikerr; 07-07-17 at 20:59.

  2. #2
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    Easiest way to do it is to fire 5 or so factory rounds and measure them with a Hornady headspace comparator.
    For gas guns, bump it back between .003 and .005.

    Drop in the .223 case gauge to verify headspace.

  3. #3
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    You must have been replying while I was measuring my cases and editing my OP. Your suggestion to bump 'er back .003 - .005 falls right in line with my thinking.

  4. #4
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    I neck size bolt action match brass and bump the shoulder back 0.002" for general bolt action ammunition. I bump the shoulder back 0.004" for semi-automatic reloads.

    Depending on what brand cases you use, you may have some spring back after cycling through the resizing die. If you are using mixed headstamps watch your brass headspace closely to ensure you are getting consistent 0.004" set back.
    Train 2 Win

  5. #5
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    I just always sized until the cases chambered easily and went from there. Never heard of the shoulder dimension affecting whether the bolt cycles--thought that was chamber pressure.
    PS: 0.556 is a large caliber. I think you moved the decimal point and meant 5.56...

  6. #6
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    You are correct about the shoulder not affecting he bolt cycle. I meant that the round would not chamber. Good catch!!

    Yea, the decimal is kinda important to put in its proper place. Wish it was one place to the right on my paycheck.

  7. #7
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    Easiest way to produce in-spec 5.56NATO ammo is to invest $25.00 or so in a good .223/5.56mm case gauge. Set your resizing die to bump the shoulder back until the case rim is just even with the low step on the case gauge. I've found that brass with the shoulder to rim length any longer, even .001"-.002", is not going to function as well as brass that is bumped back to the minimum length for head space. Most factory ammo that I have case gauged also aligns with the low step on the case gauge.

  8. #8
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    I use rcbs precision micrometer to set my resizing dies for correct shoulder set back. I can record the measured average number for five cases in order to document barrel wear.

  9. #9
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    I prefer to size to a case gauge as well. In my experience there's too much variation in how much setback you get based on brass lot, age, number of firings, headstamp, etc. to go for a specific value with any statistical confidence. This is really important for loading for a semi-auto because there aren't many things that will choke your gun faster and more frequently than a case that hasn't been sized back far enough.
    B.A.S. Mechanical Engineering Technology

  10. #10
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    Quote Originally Posted by Eric D. View Post
    I prefer to size to a case gauge as well.
    Absolutely. Anything else is nuts... at least for setting a die up for a Semi auto. I try to take a five case sample since not all pieces of brass will have the same "spring back".
    "What would a $2,000 Geissele Super Duty do that a $500 PSA door buster on Black Friday couldn't do?" - Stopsign32v

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