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Thread: Head space gauge for M1 30 cal carbine

  1. #1
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    Default Head space gauge for M1 30 cal carbine

    I think the head space is to much on my carbine and looked for a head space gauge but they aren't listed on Brownells. Does anyone make a head space gauge for the carbine?

    If the head space it too much I have to cut the barrel shoulder but that will move the gas port back. Will that be a problem?
    It's only ink and paper

  2. #2
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    there are many places that have headspace gages, Brownells is not the end-all/is-all for gun stuff.
    I just googled that and got a lot of options.

    I dont understand why you so worried about the gas port when cutting back the shoulder on a semi-auto.
    You will have to likely clock the barrel around 1 full thread, to get the shaped barrel to align, the sights to line up, and all the rest.
    My guess is to just install a new barrel IF in fact you got large headspace.
    Maybe try different bolts, you might find a longer bolt.

    What is it doing that makes you suspect headspace?
    the carbines are a pretty sloppy little gun as is, not a truly tight action, but quite serviceable.


    here is the chamber dimensions, scroll down pg 91

    https://saami.org/wp-content/uploads...sting-Copy.pdf

  3. #3
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    A longer bolt has no effect on headspace. The bolt can go no further than the face of the barrel. If the chamber has been reamed too deep, you have to shorten the barrel. The .30 carbine head spaces on the front of the case. If you shortened the barrel by one thread, I imagine your main problem would be the length of the operating rod. Hard to say, it's been 50+ years since I played with carbines. Nice little gun, always wanted one in a .17 conversion.
    Last edited by Corbettprime; 06-03-2019 at 09:15 PM. Reason: Adding to it.

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  6. #6
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    Default

    I did some searching about head space on the carbines and found most just use different bolts until the head space is acceptable. I also finally found a source for a head space gauge.

    I fired my carbine the other day and noticed that the case had a crack around the base of the case with bulging. The head was at a slight angle and the cap was flat on one side and sticking out on the opposite side. Then I fired two rounds of the same ammo and they showed signs similar to the first case with a less crack. Then I fired two LC 73 rounds and they had slight bulging at the base, no cracking and the cap and head were not distorted. The bad ammo are reloads I bought long ago and I will discard them. I still want to check the head space and found a few suggestions of how to do it without a head space gauge.

    Corbettprime, are you saying when the bolt is in battery the face of the bolt is in contact with the face of the barrel? If so then that means the recess for the head of the case determines the distance between the face of the recess to the shoulder of the chamber at the mouth of the case.

    If that is true then I could trim the face of the bolt to get the correct head space. I will do some experimenting to see if the face of the bolt touches the face of the barrel on an empty chamber.
    It's only ink and paper

  7. #7
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    can you post a picture of the cracked cases, the bulge you speak of?
    bulged cases are generally very high pressures, or enlarged chamber,
    cracks can come from very old cases, reloaded too many times

  8. #8
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    Quote Originally Posted by Carld View Post
    I did some searching about head space on the carbines and found most just use different bolts until the head space is acceptable. I also finally found a source for a head space gauge.

    I fired my carbine the other day and noticed that the case had a crack around the base of the case with bulging. The head was at a slight angle and the cap was flat on one side and sticking out on the opposite side. Then I fired two rounds of the same ammo and they showed signs similar to the first case with a less crack. Then I fired two LC 73 rounds and they had slight bulging at the base, no cracking and the cap and head were not distorted. The bad ammo are reloads I bought long ago and I will discard them. I still want to check the head space and found a few suggestions of how to do it without a head space gauge.

    Corbettprime, are you saying when the bolt is in battery the face of the bolt is in contact with the face of the barrel? If so then that means the recess for the head of the case determines the distance between the face of the recess to the shoulder of the chamber at the mouth of the case.

    If that is true then I could trim the face of the bolt to get the correct head space. I will do some experimenting to see if the face of the bolt touches the face of the barrel on an empty chamber.
    One factor to remember is that the face of the bolt may or may not be in contact with the barrel, this is not the key factor. Head space is governed by the rear of the locking lugs to the front of the chamber. Lapping in the locking lugs will increase headspace (minutely). Swapping out bolts will work, as you might find one with just a little longer distance from rear of lugs to bolt face. Like I said, its been a long time, but I seem to remember the carbine had a recessed face. If you trim the very front of the bolt, it will make no difference, BUT if you trim the recess where the head of the cartridge rests, you WILL increase headspace. There are three headspace gages: go, no-go, and field. If the bolt closes on a field gage the weapon is too unsafe to use. You could get by with a no-go, if the bolt closes on it, headspace is excessive. You can get a copy of Vickery's book on gunsmithing from Hamilton Book sellers for under $10.

  9. #9
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    Brownell has manson go & no go gauges for $35. Cheap insurance

  10. #10
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    Case bulging is caused by an oversized (large ID) chamber, not headspace. Cracks near the case head, or "case head separation", is usually caused by excess headspace but can also be caused by bad brass.

    You keep referring to a "cap", do you mean the primer?

    It's common on old military rifles to swap bolts or bolt heads around to achieve the correct headspace, and that is the correct solution if your rifle does have excess headspace. You cannot machine your bolt to reduce headspace, don't start cutting on your bolt if you don't understand what it's doing.

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