The No-Gage Headspace Test

Here’s more wisdom from Roy F. Dunlap in Ordnance Went up Front. In this case, he’s noting that captured rifles often have mismatched bolts, and headspace may be an issue. But you might not have the gages you need for some oddball foreign round. How do you know it’s okay? He provides a quick and dirty method for checking bolt gun headspace. (We have separated his 1940s wall-o-text paragraphs into shorter ones for readability’s sake).

Not many gunsmiths have headspace gages in foreign calibers, but a practical test can be made with live ammunition. At least 10 clean loaded, unmutilated cartridges, preferably military cartridges of the same type and origin should be used.

The bolt must be stripped of all parts except the extractor collar, which should never be removed, and the magazine spring and follower removed. The chamber should be polished clean with cloth, and if any trace of rust is observed, clean it out with steel wool or even crocus cloth. Chamber, bolt head and lugs and the locking lug recesses in the receiver must be absolutely clean and dry, without any oil whatever. Any burrs on the bolt face around ejector cut or firing pin hole should be removed.

With the bolt and receiver ready, the cartridges are placed in the chamber one at a time and the bolt closed very gently, the knob of the handle held loosely between thumb and forefinger. Half of the cartridges should offer no resistance to the bolt’s complete closing, and two or three cause slight effort to seat fully. If the bolt flops shut on all the cartridges available, or will not close at all on any, do not attempt to use the rifle but send it to a responsible gunsmith for a thorough check with legitimate gages. consider whatever the charge is in the light of an insurance premium.

If the bolt is “felt” To close on at least one out of every five cartridges tried, headspace may be considered safe enough for test fire at least. Some idea of the headspace distance range maybe gained by using the cartridge on which bolt closes easiest and placing as many .001″ shims as possible between bolt face and cartridge base. If the tolerance is over .008″, ‘t’aint so good – get it checked with gages.

You can see, then, why it’s only really possible to do this test as described with bolt action firearms.

He goes on from there to describe causes and consequences of too little or too much headspace, and even provides schematic instructions for making one’s own “real” headspace gages — which he notes, is quite a tough proposition.

27 thoughts on “The No-Gage Headspace Test

  1. Cap'n Mike

    Very interesting.
    Is this something he wrote for guys bringing back captured weapons or something that U.S. Military armorers would have needed to do.

    Reply
      1. Larry Kaiser

        I just ordered it on interlibrary loan. It will only cost me the postage to get it to my local library. If you just want to read the book and can get around it in 3 weeks you might look in to this option. Many library systems will let you order the book on line and call you when it arrives. I did not look up the cost of a copy but I am sure it is much more than the $4/5 bucks it will cost through interlibrary loan.

        Reply
        1. Alan Ward

          Ditto on the ILL. I often get books this way that are otherwise unobtanium.
          Our whole province is linked including all post secondary school libraries.
          Best part is this system is funded through property taxes so there is no out of pocket cost to me. It gives me some small measure of contentment to know that the rich liberal in his big mansion is disproportionally funding my gun porn addiction.

          Reply
  2. Larry Kaiser

    I went on Amazon and found new, leather bound copies for around $40 including the postage. If I was a few years younger and my house was not already filled with books that is the way I would go. As it is I am trying to find homes for the books I have.

    Reply
  3. Slow Joe Crow

    What about using plastigage on the base of a known good round? It’s considered reasonably accurate for engine bearing clearances so it should work similarly for “in the ballpark” head space estimates.

    Reply
    1. Hognose Post author

      Excellent idea, probably not available to Dunlap in 1948 or do.

      ETA: exactly that.

      PLASTIGAUGE® had it’s beginnings in the English Royal Naval Dockyards of the 1950’s. As higher specification and higher performance military equipment came into service, there was a demand for extremely accurate and easily reproducible methods for measuring bearing clearances and the clearance between hidden surfaces.

      http://www.plastigaugeusa.com/about.html

      Thought it was postwar stuff. But yeah, that’s a great improvement on his idea.

      Reply
  4. RH

    My grandfather was a collector of surplus rifles, and he used scotch tape on the base of the cartridges. Each layer would add about .004″-.005″. One layer was acceptable, two was no-go.

    Reply
  5. Dyspeptic Gunsmith

    OK, a little bit about headspacing and gages:

    1. The difference between a “go” and “no-go” gage is typically 0.004″. There are some cartridges where the difference is a bit more, but I cannot think of any cartridge where the difference between “go” and “no-go” is more than 0.006″.

    2. When I’m using gages to headspace a chamber, I’ve made sure that the chamber is clean, clean, clean. Make sure that the gages are clean, clean, clean.

    It pays to know where your datum is on a particular cartridge. On a non-belted, bottlenecked cartridge, the headspace datum will be a point somewhere on the shoulder of the case.

    On a rimmed cartridge, it will be the front of the rim. On a belted case, it will be the front of the belt. On a straight-walled case with no rim, it might be the lip onto which the mouth of the case will abut when the action is closed.

    These datums help you pay attention to what needs to be clean, clean, clean. 0.004″ is the thickness of a piece of typical laser printer paper today, or a sheet of notebook paper. It ain’t much. A piece of household aluminum foil is typically 0.001″, if you’re seeking a thinner, commonly available measure. A ZigZag rolling paper is about 0.001″, but I typically use those only for touching off on a mill.

    3. STRIP THE BOLT. If you don’t strip the bolt of any extractor, ejector, etc – you’ll likely get false or misleading readings, because you won’t be able to…

    4. Drop the bolt. When I’m checking the headspace on a bolt gun, I have the action & barrel horizontal, and the top of the action is pointed up. The bolt is stripped, per the above.

    I insert the GO gage. I put in the bolt, gently close it, then I drop the bolt handle. The bolt should close under its own weight.

    I open the bolt. I give a little puff of air into the muzzle whilst holding my finger over the back of the gage to prevent it from flying out of the chamber. Do not allow gages to hit the floor – they’re precisely ground, and even one little nick can affect their sizing.

    I pull the GO gage. I put in the NO-GO gage. Again, I gentle close the bolt, then drop the handle. Now, the bolt handle should NOT drop all the way – ie, the rifle should not go completely into battery.

    If you can close on a GO and not on a NO-GO, this is a properly headspaced gun.

    5. Uh oh! I can close the bolt on a NO-GO. Is the rifle unsafe to fire?

    Maybe. Now you’d need a FIELD gage to check “just how far out of spec is the chamber?” Obtain and insert a FIELD gage, and attempt to drop a stripped bolt on the chamber. If you cannot, then the gun might be OK to shoot, but it will be hard on brass and you should shoot only new brass in that chamber.

    When there is too much headspace, it means that the case head won’t be supported for the extent of that excess space, and your brass will tend to grow when fired. A gun with more than 0.004″ of room for the case to expand rearward will tend to need full length resizing and annealing of the brass to reload it – you’ll be working the brass hard to put it back into place.

    The other alternative, of course, is to not re-size your brass at all. OK, you have a rifle with 0.004 or slightly more headspace. Shoot your loads, then neck size only. Don’t bump the shoulder back more than a skosh, leave the brass as unmolested as you can. Once the brass has grown to fit that rifle’s chamber, it will work fine – in that rifle. It might not chamber in another, to-spec chambered rifle. It probably won’t chamber in a semi-auto. In a bolt gun, you have terrific camming force with which you can shove a round into the chamber.

    Whatever you do, do not try to close the bolt on a gage the way you would a cartridge. You’ll wreck something – maybe your gage, maybe your chamber. The bolt face will usually be hardened, and as you rotate the bolt down, you’ll see some damage to the rear of your gage – if the bolt doesn’t rotate your gage in the chamber, that is.

    Always use a set of gages from the same manufacture; ie, don’t have a GO gage from PTG and a NO-GO from Manson or Clymer. Pick one manufacture for a set of gages and get them as a set.

    Headspace checking a rifle per the instructions given by Dunlap is OK. That’s not how I’d ever check a new chamber in my shop. The above is a good field expedient on an existing rifle, where you know that the barrel tenon is the right length, the extractor cut(s) (if any) are the right depth already, etc. For new work, use a set of real gages.

    Reply
    1. Dyspeptic Gunsmith

      Forgot to add:

      If you can close an action on a FIELD gage, then it isn’t safe to fire. You don’t know how far beyond FIELD (which is typically GO+0.008″) the chamber is. It’s just “too much.”

      Reply
    2. whomever

      “Always use a set of gages from the same manufacture; ie, don’t have a GO gage from PTG and a NO-GO from Manson or Clymer. Pick one manufacture for a set of gages and get them as a set.”

      Dyspeptic: that’s common, dare I say universal, advice – but I’ve never heard anyone explain why. No one says you have to buy your ring gages or gage blocks or pin gages from the same manufacturer. Indeed, cross manufacturer repeatability is precisely the point of gage blocks et al.

      What’s different about headspace gages? Yon need the shoulder at the correct angle and correct distance from the base. What goes wrong with the geometry when you use a Brand X GO and a Brand Y NOGO?

      Reply
      1. Dyspeptic Gunsmith

        What you’re measuring when you have two headspace gages is a relative difference between the two. eg, the difference between most GO and NOGO gages is 0.004″.

        If your two gages have been ground such that they’re going to contact the shoulder cone in exactly the same place, then you’re going to get the results you want. If, however, one manufacture makes their gages to contact the shoulder in one area, and another makes their gage to contact in a different area, it is possible that you won’t get the results you’re seeking.

        If we were talking of gages such as gage pins, then buying or making a set from two (or more) different manufactures is just fine. As long as they’re all running comparable metrology equipment, they’re going to all be conforming to “class ZZ” or “class X” allowances/tolerances and you’re good. In this case, we’re talking of only two conforming measurements: the diameter of the pin, and the roundness of said pin.

        But when you get into gages that are going to mate up to a surface like the shoulder in a chamber – well, now I want to know that both gages (or all three for a full set) are being measured for their relative differences against the same conformance standard in someone’s lab or QC bench.

        Reply
        1. whomever

          Thanks. I guess I still don’t get it, alas. It just seems like getting two fairly simple features right – the shoulder angle and the base to shoulder distance – ought not be all that hard. Consider a thread gage – one manufacturer’s Class 2A thread gage is supposed to be interchangeable with another manufacturer’s. Think about how many features have to be right on a thread gage – pitch, thread angle, major dia, minor dia, the shape of the thread crest and root, etc. And for a thread ring gage, you have to hold all those on female threads! And yet, with all that, the gages are interchangeable between manufacturers.

          I wish I worked at Brownells or somewhere that had a pile of headspace gages. It would be fun to put one of the female cartridge gages on a surface plate and drop in a bunch of headspace gages from different manufacturers and measure the heights.

          Reply
          1. Dyspeptic Gunsmith

            One of the reasons for that interchangeability of thread cutting tools and gages to check them is that NIST (and other standards bodies like ANSI) have the reference standards for threads. When you get a thread gage (ring or plug), you can get gages that have traceable standards back to the master standards at NIST, ANSI or ISO.

            This is the same sort of thing as regular measurement standards. How is it that we can all measure linear things to a very high level of precision and repeatability? Because we have worldwide master standards for “what is a meter?” and so on.

            eg, let’s talk about thread ring gages, which as you point out, have lots of fiddly little issues they have to attend to. Well, how do you know that female thread is correct in the ring gage?

            You put in a master male thread plug gage. When you use thread ring gages, you have to own a master male thread gage set to check the female ring gage. You then need to adjust the ring gage on the plug gages to get the correct results.

            OK, so where’s the master standard for a chamber, either the reamer side or the actual chamber? Where’s the traceable metrology standard for chamber reamers?

            (insert sound of crickets right about here)

            NIST maintains no references on chambers, either reamers or chambers. I don’t know if CIP does.

            The only references we have for chambers are SAAMI and CIP, and then all we have are chamber drawings, first from SAAMI, and then books of more chambers like Dave Kiff’s book of chamber prints. There aren’t, to my knowledge, any “master standard” of chambers or reamers, other than perhaps the military chambers (5.56 and 7.62), and then those standards probably aren’t available as reference standards for the public. They’re used to check contractor submissions or program compliance.

          2. whomever

            “have the reference standards for threads.”

            Are you saying that someone (NIST or the people in Paris with the standard kilogram) has a physical master standard for, say, 1/4-20 class 2A standard threads? My sense was that thread masters were defined by pitch diameter, etc, and that the definitive measurement of pitch diameter was by measuring over three wires. FWIW, that seems to be what my metrology reference books imply, and what a bit of googling implies. If you have a reference that says NIST has a collection of thread plug gages that are absolute standards, in the sense that the standard kilogram is by definition the kilogram, I’d love a pointer.

            But back to headspace. Let’s imagine a notional cartridge, which has a 45 degree shoulder and headspace is defined as the distance from the base to the place on the shoulder where the diameter is 0.5000, and that distance is 3.0000 inches, and allowable headspace is 0.010 (I know, pretty loose!). So perfect headspace gages would measure, say, 3.0000 for GO and 3.0100 for NOGO.

            Let’s further assume that all our manufacturers are really good, and make their gages to zilch tolerance .000001 or whatever, so we can disregard those errors. I think you’re saying that if we measure Alpha Company’s gages, we might find them to be 3.000 GO and 3.010 NOGO, but Beta Inc’s gages measure 3.003 and 3.013, i.e. each company maintains the same difference between GO and NOGO, but the actual lengths vary.

            And so, if we measure an actual rifle with an Alpha GO/Beta NOGO, we’d be looking at a 0.013 diff, whereas a Beta GO/Alpha NOGO would differ by only 0.007 (3.010-3.003). And I agree that’s a little weird, but I’m still not getting something.

            What we’re trying to tell is:
            1)is my chamber too short, so rounds won’t chamber, and
            2)is my chamber too long, so I’ll get head separations
            I get that if the cartridge manufacturer uses a short gage and the rifle manufacturer uses a long one then I’ll see problem #2, and conversely will see #1 if the cartridge manufacturer has the long gage and the rifle manuf. the short one.

            But in answering the question ‘will my rifle work OK with generally available ammunition’, I care about my chamber relative to the ammunition dimensions. If Acme Ammo is producing 3.000 inch ammo, then I will be stretching it 0.013 if the Beta NOGO just fails to close. That’s true whether I did the GO checking with the Alpha 3.000 or Beta 3.003 GO gage.

          3. Dyspeptic Gunsmith

            NIST will test a manufacture’s (ie, a gage maker’s) master gages to NIST standards. The provide a service to anyone who wants to send them a ring or plug gage for threads (as well as some other types of machined parts).

            Here’s the services and prices that NIST will offer/charge for “complex dimensional standards:”

            https://www.nist.gov/calibrations/complex-dimensional-standards-calibrations

            NB the other national metrology labs listed on the page.

            NIST will then provide “traceability” references back to the standards (physical or created), machines and calibrations they used to verify a manufacture’s master gages.

          4. Hognose Post author

            Calibration, traceability and documentation are big deals in manufacturing. Most home and small ‘smiths don’t know just how long and hard the battle for interchangeable parts was. In the 18th Century, it was thought to be an unattainable Holy Grail. In the 19th, it existed more in theory than in practice. (Parts from Cvil War icontractor rifle-muskets might or might not fit Springfields, and one Springfield’s trigger might not fit another, for instance). Well into the 20th Century, manufacturers shipped national armies firearms with non-interchangeable parts. Gradually, both design and manufacturing tightened up. But even today, despite the example of the AR, guns != legos.

          5. whomever

            DP: NIST will test thread gages, but I don’t think they are comparing them to a master gage.

            I.e., for distances, your measurements ultimately are derived from the standard meter (back in the day) or optically measured standards (since the meter is now defines as N.N wave lengths of such-n-such wavelength of light. Either way, you calibrate your master gage blocks, and go from there.

            So if you want NIST to calibrate a gage block, they compare it to their master blocks. But If you ask them to certify the pitch dia of your thread gage, I think they are going to measure over wires; that measurement will be calibrated against gage blocks. Or, these days, maybe they use some fancy measuring machine – but I don’t think they compare your gage to a reference master thread gage, they just measure your gage over wires. I could be wrong, though, and always like to be educated.

            But, back to headspace gages: it’s just not hard to produce things to a couple of tenths these days. The ‘H’ limits for taps, for example, span .0005, and taps cost less than headspace gages, and are a lot more complex geometry. You’d think that someone (SAAMI??) would specify headspace dimensions, and people would make them to those dimensions within .001 – which is plenty of accuracy for the task.

            Like I said, it would be fun to measure a bunch from different manufacturers, and get the bell curve for GO and NOGOs across manufacturers!

            Apologies if I’m coming across as pedantic; I’m fascinated by metrology, and as Hognose points out, developing reliable inter-factory gaging is no small contributor to the wealth we enjoy today.

    3. Hognose Post author

      DG, there’s a lot more on headspace in Dunlap; I pulled out this bootleg headspace checking technique because I’d never seen it anywhere else.

      Reply
      1. Dyspeptic Gunsmith

        Oh, it’s an old gunsmith trick, alone with measuring spent cases with a mic or calipers to see how much the cases have elongated during firing from spec chamber sizes.

        Much is made of headspacing to “0.0” these days, because everyone wants allowances and tolerances closed up to minimums. Most wartime bolt gun allowances on the action, bolt, etc would end up with perhaps 0.002 to 0.004 variation (plus or minus) showing up in headspace if you shuffled a bolt between guns, either way. If the bolt you shuffled from one rifle to another ended up in the headspace being too tight, they’d sometimes just keep shuffling until it was easy to close the bolt on a new-issue round in another action. Most of the time, even with shuffled bolts the headspace wouldn’t violate a FIELD gage.

        Many bolt guns with the sort of pressures wartime issue ammunition was using could tolerate the headspacing being out of spec up to 0.010″ or perhaps a skosh more without case failure. This was possible in part because their pressures were nowhere near the low to mid-60K PSI region that many new cartridge designs are sporting today. Most ball ammo pressures in WWII were about 55K PSI and lower, with the .30-06 being about the highest pressure of the lot and many of the Euro-weenie cartridges being in the high 40K range.

        Once you’d get to reloading your ammo, however, now you wanted to know your headspace was where it should be.

        Reply
    4. John Distai

      When should you headspace a bolt action? Is this only for rifles that have been disassembled and reassembled, or should you periodically headspace a factory rifle that has not been otherwise disassembled?

      After how many full-length resizings would you recommend brass annealing?

      Reply
      1. Dyspeptic Gunsmith

        Well, I headspace any time there’s a change to the bolt, barrel or chamber, or if a customer is complaining about how his brass develops that bright line forward of the web that indicates incipient failure after “too few” reloadings. On something like a Remington or other action where there’s a washer between the barrel and action (ie, what Remington calls their recoil lug), if that gets ground or replaced, then you’d need to headspace again.

        If your rifle has retained the bolt it was manufactured with, and there has been no change to the barrel, you’ve never unscrewed the barrel, you’ve never dropped a reamer in the chamber… and you knew it was headspaced properly when made, then there’s no need to check the headspace again until you are going to change any of these issues.

        On an AR that has lots of wear, you want to check the headspace if you’re replacing the bolt or the barrel extension – or you’re dropping in a new barrel and you’re going to use an existing, ie used, bolt. In an AR, I strip the bolt, put the bolt into the carrier with the cam pin, put the GO or NOGO gage into the chamber and push down on the bolt carrier. If the bolt rotates fully into battery, then that’s closure.

        Reply
    5. John Distai

      If I have fired rifle brass, can I use the LE Wilson Case Length Headspace gauge to determine if the rifle has the correct head spacing? I have one of these gauges, but I’ve been using it to check to see if fired or resized brass fits in it and to check the neck length for trimming. Am I using it incorrectly?

      Reply
      1. Dyspeptic Gunsmith

        Yes, if you’re using the end with the steps to discover whether you’ve resized your shoulder too far back. If I recall how these gages work, if you drop in a piece of clean, fired brass, and the head is proud of the bottom of the Wilson gage, then your chamber headspace might be a bit long. You’d need a precision measurement tool (depth mic, mic or calipers) to tell you how much, tho.

        Reply
  6. Visa

    Did the headspacing with assorted spacers and a caliber. Worked OK and shoots safe and accurate, this beaten up Lee-Enfield 1914 BSA with German (counterfeit?) capture marks.

    Reply

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