Headspace gauge logic algorithm

Discussion in 'Black Rifle Forum' started by cciman, Jun 9, 2020.

  1. cciman

    cciman MacGyver

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    Check me on this logic path. If it Closes on:

    GO gauge -if it closes, it is safe

    NO-GO- it is not a new rifle, but not necessarily "unsafe"

    Field- if it closes, it is not safe.

    If above is correct, and I want to minimize buying parts, and I want to check sharing BCG between ARs, I could just buy one- either the Go or Field gauge. Why need more than one?

    Why is one better than the other? Why need a set?
     
  2. cciman

    cciman MacGyver

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    Probably should be in gunsmithing, rather than here.
     

  3. Big Bird

    Big Bird NRA Life Member

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    You only need the Field gauge. Most military grade chambers are cut a little long to provide for reliable feeding and extraction in the field under field conditions...meaning fouling, dust, mud, water etc. I've helped barrel many ARs and M1As and even in match grade rifles and DMR guns sold to the military the bolts will go home on a No-G0 gauge if your press on the back of the BCG. A tight chamber in a semi-auto rifle is mistake.
    You don't need a "go"gauge unless you are actually cutting chambers on a lathe. The only gauge you really need is a field gauge to check for excessive headspace. The rest are unnecessary for the hobbyist playing with already machined parts.
     
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  4. faawrenchbndr

    faawrenchbndr CLM

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    Keep in mind, you need to strip the bolt to properly measure headspace
     
  5. cciman

    cciman MacGyver

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    Please explain fully. What do you mean by "strip the bolt"? Why?
     
  6. faawrenchbndr

    faawrenchbndr CLM

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    Extractor needs to be removed to properly measure headspace. I learned that the hard way, had problems, asked the question and found out it needed to be removed
     
  7. ThomasM4

    ThomasM4

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    Both replies are correct field gauge and the bolt needs to be stripped unless you get the military amours gauge specifically for the ar15 its cut to clear the extractor and ejector.
     
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  8. Jim Watson

    Jim Watson

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    If you horse the bolt closed like you were chambering a round, you will not get a good headspace reading, the bolt should be closed with fingertip pressure only. I don't know how to do that in an AR but it is very obvious in a bolt action.
     
  9. dkf

    dkf

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    Just buy a set of Manson "Down and Dirty" headspace gauges. You get a go, no go and field gauge for $75. If you are having issues with a rifle or just want to double check everything, the whole set comes in handy.

    FYI if the bolt won't close on a go it doesn't always mean the gun won't run. With AKs you can have some that won't close on a go or be very hard to close. Then as the round count goes up they will close on the go. I like to set my ak barrels up so they are tight on the go. The ammo is obviously shorter than the go.
     
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  10. garya1961

    garya1961

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    If you are building a new rifle you want the headspace to fall somewhere between go and no-go. If you are checking one already in use the field gauge will do.
     
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  11. cciman

    cciman MacGyver

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    Spoke to Forster, on the phone.

    Field gauge (otherwise known as MAX for 5.56Nato) is the gauge for my purpose. Thanks.
     
  12. Big Bird

    Big Bird NRA Life Member

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    This is correct! Faawrenchbender is also correct about stripping the extractor off the bolt to use the gauge. Though a Milspec Field Gauge will have a cutout on the base of the gauge to provide clearance for an installed extractor and make stripping the bolt unnecessary. But either type of Field Gauge will work as long as you understand the proper procedure.
     
  13. ThomasM4

    ThomasM4

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