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Thread: AR Headspace

  1. #1
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    AR Headspace

    Gentlemen I am in need of some advice. I believe I have a headspace problem on my NM AR. I am getting pressure signs at very low loads, flat primers at as little as 22.4 grains of Varget, CCI small rifle primers, and 77 SMK's. At 22.8 grains I start to get light extractor and ejector marks. At 24 grains (normal load) I'm getting deep ejector marks. All spent cartridges have some degree of sooting. If I use the depth gauge of my calipers to measure the step at the end of the BCG / upper reciever it measures .012", taking same measurement while using a wooden dowel to push on the bolt face that number increases to .021" which is a difference of .009". I tried this measurement on another upper receiver and BCG I have and with or without the dowel it remains at .012". Swapping BCG's from one to another the measurements are the same, meaning the BCG that measured .012 in the second case will measure .021 in the NM upper. I can also see the bolt moving rearward when pushing on it with the dowel so it's not cam pin wear (which I also measured and changed out, I was thinking it could possibly be a stack issue). When I use my Hornady cartridge headspace gauge all spent casings measure 1.46" which is what they have always measured out of this rifle. I also tried the only factory ammo I have which is some LC 55 grain which yields, you guessed it, flat primers and ejection marks. I do not have access to a set of headspace gauges but am about to call Clymer and hope they have a set in stock.
    Any suggestions will be greatly appreciated.

  2. #2
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    You should magnetic particle inspect your bolt and barrel collar for cracks, particularly if your gauged headspace seems correct. A cracked lug in either part would most likely be the shortest headspace tolerance pair and your headspace would then open up to the next shortest tolerance pair under firing loads.

    GI spec AR's typically experience lug cracks around 3,000 rounds fired and become common around 5,000 rounds fired.

  3. #3
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    Thanks for the response, is it cost effective? Or is a new barrel and bolt a better option? I found a couple places in SE Michigan that do that just not sure what a two piece inspection would cost. Do you know of a place that does this as a gunsmith service? Thanks

  4. #4
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    Quote Originally Posted by Double_Tap View Post
    Thanks for the response, is it cost effective? Or is a new barrel and bolt a better option? I found a couple places in SE Michigan that do that just not sure what a two piece inspection would cost. Do you know of a place that does this as a gunsmith service? Thanks
    May be able to arrange it. PM me. A good first step is examination under a binocular microscope. That I can do for certain.

  5. #5
    Quote Originally Posted by 10x25mm View Post
    May be able to arrange it. PM me. A good first step is examination under a binocular microscope. That I can do for certain.
    Where you at ? I have go no go gauges if you need to check it . I wouldn't be shooting it if you think the headspace is off

    Sent from my SM-G960U using Tapatalk

  6. #6
    MGO Member Moleman-'s Avatar
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    I agree You likely have excessive headspace, but you're actually measuring bolt clearance which isn't exactly the same thing. If for whatever reason when they were making your barrel if the barrel extension tenion length on the barrel were closer to .611" and not the max of .620" you would have the extra bolt movement mentioned even if it were headspaced tight. Since there is a question I'd get a headspace gauge set. Manson Reamers here in MI has a "down and dirty" 3 gauge set for $70. Measures the same as the more expensive gauge sets but they don't polish the non critical areas.

    What is the COL you're using for that load, case water capacity, and how far off of the lands are you?

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