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    Legacy Member tenOC's Avatar
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    Head spacing with only a FIELD gauge.

    I've read where people state they only use the FIELD gauge when checking head spacing. Is that really common?
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    Yes, particularly if you have a used firearm. In all likelyhood a used firearm does not have too short a chamber unless you have changed the barrel. So for a used firearm you really want to know that the headspace is within acceptable limits.
    When they tell you to behave, they always forget to specify whether to behave well or badly!

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    Legacy Member tenOC's Avatar
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    I have one that closes easily on the NO-GO. But doesn't on the FIELD.

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    Legacy Member JackP's Avatar
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    Just know that the standard for the maximum head space has evolved over time. Early TM's specify 1.300". Later TM's specify 1.302". Modern commercial field gauges (Forster) are 1.299". So it makes a difference which "Field" gauge you use. I recommend the more conservative 1.299" gauge. If it barely closes on this gauge you may still be okay according to the TM's, but be careful.

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    Legacy Member tenOC's Avatar
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    Ok. My Forster Field does measure that 1.299.
    The No-Go is 1.297 and the Go is 1.291.

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    Quote Originally Posted by tenOC View Post
    Ok. My Forster Field does measure that 1.299.
    The No-Go is 1.297 and the Go is 1.291.
    Ten, are they also stamped with those same numbers? I guess mine are older than I thought - without pulling them out, I remember them as 1.290, 1.296, and 1.302. - Bob

    EDIT: Correction - My field gauge is stamped 1.299 - not 1.302.
    Last edited by USGI; 10-10-2019 at 10:24 PM.

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    "...only use the FIELD gauge..." That doesn't actually check the headspace. It only tell you the thing is at less than max.
    Yours closing on the No-Go but not the Field indicates it's safe to shoot, but is close to max.
    "...My Forster Field does measure..." Headspace gauges do not 'measure' anything. They only tell you if the thing is within tolerance or not.
    Graf's shows the Forster No-Go gauge to be 1.296". Not that it matters. They don't list the other 2 gauges.
    Spelling and Grammar count!

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    Quote Originally Posted by Sunray View Post
    "...My Forster Field does measure..." Headspace gauges do not 'measure' anything.
    I believe he's saying he measured the gauge - that's why I asked if it was stamped. - Bob

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    TM9-1276_1947
    http://www.90thidpg.us/Reference/Man...-1276_1947.pdf

    Shows 1.290 (GO), 1.295 and 1.300 (NO GO) ... page 25.

    Section 15 on pages 17-18 reads:
    15. Headspace Gauging
    a. The headspace of a carbine is the distance between the shoulder
    of the chamber and the face of the bolt when the bolt is in locked position. If headspace is insufficient, the bolt will not fully lock behind the
    cartridge without being forced. If headspace is excessive, the cartridge
    will have too much play in the chamber when the bolt is locked behind it.
    Either condition is unsafe. As component parts of the carbine
    are manufactured to close tolerances and headspace is carefully checked
    at manufacture, a variation usually is due to wear and causes excessive
    headspace to develop. However, assembly of parts with maximum
    tolerances may result in either excessive or insufficient headspace.
    b. Excessive headspace due to wear may be caused by advanced
    chamber shoulder, worn faces of bolt, worn locking lugs on bolt, or
    worn locking shoulders in the receiver.
    c. Test headspace with gauge (fig: 54), as follows:
    (1) Clean bore, chamber of barrel, and operating parts thoroughly,
    wipe dry, and inspect for metal fouling or foreign matter. Operate
    the mechanism a few times to see that bolt closes and locks smoothly
    on an empty chamber.
    (2) Retract and hang the bolt by means of the slide stop. Place the
    headspace gauge on the face of the bolt, gripped by the extractor. Be
    sure that gauge is perfectly clean and dry.
    (3) Retract the bolt slightly to disengage the operating slide stop,
    and allow the bolt to move slowly forward to the locked position so
    the gauge enters the chamber of the barrel. If the bolt locks fully on
    the maximum gauge, the headspace is excessive and the carbine unserviceable. Lock the bolt completely when the minimum gauge is used.
    Caution: Do not force, or allow bolt to close sharply under spring
    propulsion.
    (4) Pull back bolt and remove gauge.
    Note. See serviceability chart (fig. 19) for proper headspace gauge. Forward headspace gauges to an arsenal once a year for checking.

    After Re-barreling, For Proper Headspace... page 79/80..

    CHECKING CHAMBER FOR PROPER HEADSPACE. If after rebarreling, there is insufficient headspace indicated by the bolt failing to close on the 1.290 headspace gauge (41-G-199-175), the chamber must be reamed with carbine reamer (41-R-488-125).
    To correct insufficient headspace, proceed in the following manner:
    (1) Dip reamer in lard oil and place in chamber through top opening of receiver. (See fig. 55.) Turning handle slowly to the right, remove only sufficient metal to permit bolt to close on 1.290-inch headspace gauge (41-G-199-175).
    (2) Wash chips from reamer with lard oil after removing from
    chamber.
    (3) Handle reamer with care to prevent it from being chipped or
    nicked. Never turn reamer backwards in chamber, as this tends to
    dull the cutting edges.
    (4) If bolt closes properly on the GO gauge (1.290), the NO GO
    gauge 1.300 (41-G-199-200) is then inserted. The bolt should not
    close on the 1.300 gauge. If bolt does lock completely, it indicates too
    much headspace. In this case, use a maximum bolt and try the gauge
    again. If this bolt also locks completely, the receiver is not usable.
    Charlie-Painter777

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    Legacy Member tenOC's Avatar
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    Quote Originally Posted by USGI View Post
    Ten, are they also stamped with those same numbers? I guess mine are older than I thought - without pulling them out, I remember them as 1.290, 1.296, and 1.302. - Bob
    Quote Originally Posted by Sunray View Post
    "...only use the FIELD gauge..." That doesn't actually check the headspace. It only tell you the thing is at less than max.
    Yours closing on the No-Go but not the Field indicates it's safe to shoot, but is close to max.
    "...My Forster Field does measure..." Headspace gauges do not 'measure' anything. They only tell you if the thing is within tolerance or not.
    Graf's shows the Forster No-Go gauge to be 1.296". Not that it matters. They don't list the other 2 gauges.
    They're actually stamped 1.290, 1.296 and 1.299, but I measured them with a digi micrometer made in the largest Asian nation and sold at a premium from a very high end tool distributor outlet called Harbor Freight. So....there ya go. China is always right, aren't they?
    Last edited by tenOC; 10-10-2019 at 06:31 PM.

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