Help me understand "Head Space"

[F. Guffey]

If we only knew what that was maybe we could settle this . oh well I guess we will never know what those things are for

"maybe we could settle this" We? To solve a problem you must contribute. An old song written by a driver while going to California suggested "Relax your mind".

Years ago I decided to build a rifle, I received a lot of advise. I did research, every time the word 'head space' came up in a conversation and or research material a very negative phrase was repeated. 'no one knows etc.' so check often. Short chamber? How short? How close to finishing? Check often. How far must the reamer be advanced to finish the chamber? Check often so 'You' will know how far the reamer must advance to finish the chamber.

Instead of building the rifle I decided to determine why the lights went out when the bolt closed. I built tools that would allow me to determine the length of a chamber with the bolt closed. I made tools that would allow me to measure the length of the chamber with the bolt open.

Then there were unique features of millions of rifles like the 03. All that was ever necessary to check the length of the chamber from the shoulder to the bolt face was a head space gage and or the chambered ammo 'AND, A FEELER GAGE.

If we only knew what that was maybe we could settle this . oh well I guess we will never know what those things are for

There were reloaders on forums that scorned the day they ever learned to read, I did not start with "Hatcher said....". My question was "How could he miss the third lug copied from the Mauser, the Mauser third lug is out of sight, on the bottom, the 03 third lug is exposed and in front of the rear receiver ring.

What do you want to know? "What it is?" or how is it possible to measure the length of the chamber from the datum to the bolt face. I have modified go-gages into go to infinity-gages.

Just because the bolt closes and the chamber gets dark the light in the mind does not have to go off.

F. Guffey

[Gunplummer]

Sir Gilligan, You are not as messed up as you think. I have ACTUALLY ground both headspace gages and chamber reamers. Think about what you read: "Headspace is the distance between the head of the case and the face of the bolt". If you subtract the dimension of a "Go" gage from the dimension of a "NO GO " gage, that is what you have on rimless cases. As you change case styles, the reference points change for the headspace. It is totally possible to "Create excessive headspace" by resizeing your brass too short. Unbelievably, Mr. Guffy actually wrote something that made sense. There is a lot of talk being tossed around about the "Datum line" in SAAMI specs. It would be interesting to see just how many people on this thread know how that datum line is referenced. He is correct (90%), the datum line is not the same in military and SAAMI specs. That is why people think they have "Bad headspace" when they mix SAAMI spec brass with older US military and European chambers. Allegedly, the crossover between the two is exactly the same. Obviously not. There are many shades of grey when talking about headspace, but I believe you have a basic understanding, and that is what counts.

[mehavey]

Actually, its not the "datum" reference that's changed (betwixt SAAMI/Military
chamber headspace specs), but the spec itself.

To wit:

The "datum" reference for 308Win/7.62NATO is a circular hole 0.400" in diameter.
Where the case/shoulder hits/stops in that circle is the "datum" point.

The acceptable distance between the bolt face and that "datum" point in the
chamber's shoulder is between:

SAAMI:
1.630" (Must GO) - 1.634" (NOGO)** 1.638 (FIELD REJECT)
NATO:
1.6355" (Must GO) - 1.638" (NOGO) 1.6455(FIELD REJECT)

The idea is that the military has NO problem which a bit of case stretch when the
cartridge MUST be able to fit in the chamber each time, every time, no matter what


** actually, SAAMI chamber drawings have it at 1.634 - 1.640 MAX. But Commercial FIELD Gauges are cut 1.638"

[sirgilligan]

Gunplummer,

Thanks.

In reloading, I am making "generic" rounds with small base dies, so that I can shoot the ammo in my three very different rifles that are the same caliber, one a Ruger, one a CZ, and one a FN.

But I have learned from studying headspace that most are not reloading like I am, they are making custom sized cartridges that are close to the dimensions of their unique chamber. They are "filling" the chamber and closing the gaps.

I have learned that there are several variables involved.

The rifle chamber, was it cut exactly right (perfection doesn't happen often in this world), or is it a little long or a little this or a little that.

The bolt, does it lock up where it is supposed to, is it flush and square (I am not sure of the proper terms).

The cartridge, is it shaped correctly to stop forward movement where it is supposed to, etc.

I am thinking, if you put an out of spec cartridge in an out of spec chamber and they are both out of spec the same way so to speak then it is a good match.

Ideally we are putting in spec cartridges into in spec chambers and the brass doesn't have to deform much to fill the gaps and everything works better and the brass lasts longer.

Those are some of the things I have learned.

But we are still all doooooooooooooomed, optimistically speaking of course.

[F. Guffey]

I am thinking, if you put an out of spec cartridge in an out of spec chamber and they are both out of spec the same way so to speak then it is a good match.

I have a chamber that is .002" longer than a field reject length gage from the shoulder to the bolt face. 'that is out of spec'. When sizing cases for this chamber I add .014" to the length of the case from the shoulder to the head of the case. Because no manufacturer makes cases available for long chamber I use 280 Remington cases, I adjust the die off the shell holder .014" then size the case and measure the length from the shoulder to the head of the case.

I do not use cases that have excessive resistance to sizing, I use new and or once fired cases, then there is always annealing.

Next, there is cylinder brass, a straight wall case that is 2.650" long. Expensive, close to $38.00 for a box of 20.

F. Guffey

[F. Guffey]

Actually, its not the "datum" reference that's changed

I have a head space gage that is marked Government 06, the ("datum" reference that's changed) datum reference did change, SAAMI changed the reference, back to "Hatcher said". The 'measure from' in the beginning was the case body/shoulder juncture. SAAMI moved it to the shoulder, round hole of .375".

I do not allow this stuff to get the best of me, I can use any diameter that is between the shoulder/neck juncture and the case body/shoulder juncture. All I have to do is use the same diameter hole, for accuracy I do not use case friendly holes, I use sharp edges, no radius or bevels.

Wilson case gages use a radius on their 'measure from' datum on their case gages.

F. Guffey

[Unclenick]

Mahevey is correct. I'd forgotten the STANAG 7.62 drawings also use the datum as the measuring point, too. It should be, since they got the cartridge specs from us. It is only the CIP drawings that still don't use it and still work the way Hatcher did.

It is important to keep in mind the change in rimless bottleneck cartridge reference point from case body/shoulder intersect to the datum did not change the chamber size. Rather it changed where the measurement that ensures a particular size was being taken.

Originally Posted by Prof Young

"Headspace" is the size of the chamber in relation to the size of the cartridge.

That's the only part of what you said that's not quite right. The headspace belongs to chamber and doesn't depend on the cartridge case. Only head clearance is dependent on the relationship between their two sizes.

[Gunplummer]

SAAMI datum line for rimless cases is taken from where a .375 ball contacts the shoulder angle. True, the length dimension for MIL spec and SAAMI is SUPPOSED to be the same, but throw in the cartridge spec differences and there are discrepancies. When you mix chamber specs and cartridge specs is when you start to see the difference. You are not really dealing with a linear movement. I don't remember how many times someone brought me a '98 Mauser that "Failed" someones headspace gage and there was absolutely nothing wrong with it. If a gun really blew out the brass, it was "Shot-out". For many years there have been warnings in load books about the wall and neck thickness of military brass in SAAMI spec chambers. That was not old wives tales.

[Metal god]

He is correct (90%), the datum line is not the same in military and SAAMI specs. That is why people think they have "Bad headspace" when they mix SAAMI spec brass with older US military and European chambers. Allegedly, the crossover between the two is exactly the same. Obviously not.

I don't remember how many times someone brought me a '98 Mauser that "Failed" someones headspace gage and there was absolutely nothing wrong with it.

Doesn't that mean the shoulder angle in the chamber is out of spec and if not out of spec , it's different then SAAMI ?

[Unclenick]

Originally Posted by Gunplumber

SAAMI datum line for rimless cases is taken from where a .375 ball contacts the shoulder angle.

It's actually a 0.375" cylinder for chambers and hole for cases, and that number only applies to .30-06 family cartridges. It's 0.400" for 0.308 Win and 0.330" for .223 Rem and different numbers for other cartridge families. You just have to look up the SAAMI drawing and see what it is for your cartridge.

[F. Guffey]

You just have to look up the SAAMI drawing and see what it is for your cartridge.

I don't, when using a comparator I compare, something like A - B = C. Then there are all of those makers of gages they call head space gages for cases. They make case friendly gages with radiuses. And that just locks the reloader up because 'now' they have to get a standard to check the gage if they are going to 'look it up'.

As the man said as he headed off for California to work on the 'defense', "Relax your mind".

F. Guffey

[mehavey]

Re the Case Headspace Comparators (aka 'gauges')

- Take a fired case.
- Begin to resize it to where it squeezes the case sides in/elongates the total case to the point it won't fit in the chamber anymore.
- Slowly screw the sizer down until the case will just fit w/o bolt resistance
- Measure the result case headspace dimension with the comparator.
- Write it down and use it forevermore when sizing cases for that rifle.

[F. Guffey]

Doesn't that mean the shoulder angle in the chamber is out of spec and if not out of spec , it's different then SAAMI ?

No. In a cryptic way I explained that, I make gages, to make a gage I decide/pick/choose the diameter. The diameter can be any diameter that is greater than the diameter of the shoulder/neck juncture and less than the diameter of the case body/shoulder juncture.

Comparator, I use comparators. I do not have case head space gages because the case does not have head space.

F. Guffey


There is one minute difference between the last two post.

[mehavey]

...the case does not have head space

But its does have a headspace dimension. [goad, goad]

[F. Guffey]

A member of a reloading forum called SAAMI, seems SAAMI omitted the symbol for head space on the case drawings. And that just locked him up. I have no problem with it, if SAAMI says it is there, it is there. On the other hand if "SAAMI SAYS" it is not there, it is not there.

I can only imagine the conversation "Do you know who I am!?" When I want to know the length of a case from the datum/shoulder I measure, when I want to know the influence the length of the case has on the length of the chamber I measure.

It is possible to fire a case then measure, I suggest measuring first then fire. BUT! It is not possible to move beyond that because of al the jump back, snap back and spring back reloaders. Snap back, jump back and or spring back does not drive me to the curb. If it bothered me I would dial it in on the dial caliper for correction.

F. Guffey

[Unclenick]

Originally Posted by unclenick

You just have to look up the SAAMI drawing and see what it is for your cartridge.

I phrased that badly. What I meant was that IF you want to know SAAMI's datum line diameter, you will have to look it up.

As Mr. Guffey frequently points out, sizing a case for a particular chamber only involves knowing the sizes of that chamber and that case are compatible, and you can make up your own datum diameter for that purpose, as long as it makes a valid comparison. The example I've posted before is the home improvised gauge. On the .308 case it doesn't match the SAAMI datum diameter exactly, but it makes a measurement you can compare on a case before and after sizing to see how much you've set the shoulder back to know that it will fit the chamber. It is useful for setting the sizing die up, as it is possible to either under or over resize.



I think it is important to remember SAAMI is a manufacturing organization and its standards exist to allow one member company to make ammunition that fits and fires safely in other manufacturer's guns. Handloaders can choose to load separate cases for each rifle they own, just as they load to pressures and velocities their individual gun shoots well with, and most who are concerned with maximum accuracy do both. Only folks desiring to load for compatibility with all guns in a chambering, or who are chambering a rifle that needs to be compatible with commercial or military manufactured ammo, will need to worry about the exact SAAMI numbers. Nonetheless, it doesn't hurt to understand where SAAMI numbers are coming from or what they mean.

[Gunplummer]

I have dealt with SAAMI in the past, with both good and not so good results. I remember calling them about chamber specs for a reamer I was making (.22 HiPower) that was not in their book. The guy on the other end of the line told me he never heard of it. I had to pull a Guffy and use fired brass and neck/free bore specs off of compatible cartridges. It worked out fine. A lot of things got somewhat clearer since the internet. You almost had to threaten them years ago to get information and I suspect it was because they could not give you a definitive answer either. Look at all the answers on this thread. I never heard of the .400 circle scenerio. The information from the Frankford Arsenal years back said junction of shoulder and body angle. That was good enough for me. What it all comes down to IMO, is the specs for the chamber AND the case are inseparable and dependent on each other.

[Machineguntony]

I started reading this thread because I just purchased a gun with adjustable headspace. I am told that it has 2-7 clicks for headspace. I have never heard a gun with an adjustable headspace, prior to this gun.

Previously, I had never paid attention to headspace.

What are the consequences if the headspace is wrong or off?

[Gunplummer]

You will probably be sprayed with brass. I have seen guys sitting under M-2's take a lot of brass in the thighs and worse when the headspace and timing was not set right. I saw one where the cover was completely blown off. The old .30 cal 1919's were set the same way. Most other machine guns had the headspace locked in on the spare barrels. What are you buying? I guess you could say a Savage is adjustable with the jam nut on them.

[HiBC]

I recall the Browning 1919 and the M-2 require headspace adjustment.

Your MG has to be loose enough to lock up every time,easily with no interference.
It has to be tight enough to not over stretch brass resulting in case separation.

Wartime production ammo,perhaps from multiple countries, ,interchangeable barrels,North Africa to the Chosin reservoir...Its going to be within 2-7 clicks.
Test fire and go with what runs...
I'm not a gunner or a Veteran.I just made that up.

[Machineguntony]

I thought headspace only affected accuracy. Interesting.

[mehavey]

I recall the... M-2 require headspace adjustment.

"Headspace amd timing sir.... Headspace and timing...."

I still hear it in my sleep.
https://www.youtube.com/watch?v=zYFVFWDfZ14

[F. Guffey]

Check and adjust, check and adjust:

The fixation on 'THE HEAD SPACE GAGE' is a limiting factor. The go and the no-go causes mental blocks. I have gages that go from too short for short chambers to too long for long chambers. When checking a receiver and a barrel for the length of the chamber from the shoulder to the bolt face I use different length gages.

Then there is the gap created between the front of the receiver and shoulder at the end of the threads and verifying with a feeler gage.

F. Guffey

[Gunplummer]

Machineguntony--Looking at the user name, I went up to the full auto section and there you were. I worked on 240's when the Army bought them for co-ax guns in the 60 series tanks. These guns were heavier than the ground models. I know because I looked at one that some Irish Rangers had I was working with. Anyway, the clicks you are talking about are on the gas regulator. What it does is speed up the rate of fire. You can get an out of time condition, depending on the wear on the guns components. I don't remember how many clicks were on the tanker model. I do know that you could crank them up until you actually had a sheet of flame coming out of the bottom of the receiver where the empties dropped down. Like I said before, the tank model was much heavier. It has been a long time, but I believe these guns were actually manufactured in Belgium.

[Unclenick]

Gunplumber,

Yes. The Internet age has changed it all. Lot's more available.

As you can imagine, since some cartridges are narrower than 3/8", you can't find the shoulder with that number on them. Similarly, some cartridges have bullets wider than 3/8", so you couldn't find a shoulder number at that diameter on them, either. Here are some drawings from the SAAMI site. The upper is the cartridge, while the lower is the chamber. Note the line through the shoulders of both have the same diameter number, with the letter "B" following. That B is for Basic, meaning it's a defined dimension, so it has no tolerance, and is a number other measurements are referenced to. In other words, a datum.

460 Weatherby Magnum, 0.5300" datum diameter.
375 Remington Ultra Magnum, 0.465" datum diameter.
308 Winchester, 0.400" datum diameter.
30-06 (and all other 30–03-based cartridges, IIRC, including 280-AI and others, 0.375" datum diameter.
222 and 223 Remington, 0.330" datum diameter.
218 Bee, 0.2875" datum diameter.

If you go to the SAAMI site to look up individual cartridges, here, note that the site has a technical issue (which they mention) in that this page's links to drawings only work in Internet Explorer and no other browser.

Note that the tolerances in SAAMI drawings are unilateral. This is a common engineering practice where getting tolerances too large in one direction is critical to operation and the other direction is less so. Most of us are accustomed to the familiar + and – tolerance, where the value given is in the middle of the range, but that's a situation where being bigger or smaller impose equal problems. For unilateral dimensions, you give the critical extreme value and then give the tolerance only in the other direction. For example, a shaft that's too small for a bearing will vibrate and wear itself or the bearing fast, but a shaft that's too large cannot even be assembled into the bearing. Thus, too large is the critical dimension, and so shafts are specified as a maximum diameter with only a minus tolerance for proper operation.

In the case of the SAAMI drawings, lengths and diameters and inside corner radii of cartridges cannot be allowed to get too large or they won't fit in a chamber. So all those dimensions are given as maximums and the tolerance is minus only. An outside corner radius is a minimum with a + only tolerance, as shrinking an outside corner, like the shoulder and body intersect corner, can jam in a chamber. For the chamber it is the other way around, with the minimum space being critical to being able to chamber a round. So chamber linear dimensions and outside corners, like the neck and shoulder intersect, are all minimum dimensions with plus tolerances only, while inside corners, like the neck and shoulder junction for a chamber, have radii that are maximum dimensions with minus tolerances.

One funny result of this is most SAAMI cartridge maximum dimensions are bigger than minimum chambers because it is still possible, allowing for the case to expand to the width of a minimum chamber, to force a fit. However, nobody intentionally manufactures cartridge cases over minimum chamber size on purpose. That happens when they expand into a headspace that is larger than minimum, but don't come that way from the factory (remember that factories want their ammo to fit all guns). The military, though, doesn't play that game. They want to be sure there is no "squeeze" that might slow full-auto fire, so NATO chamber minimums equal cartridge maximums.

Measuring from the breech to the body/shoulder intersect is the old way used by Hatcher and, apparently the CIP, since they don't provide a datum. It works, but has two limitations. First, the body/shoulder corner in a real chamber has at least a microscopic radius, so the actual intersect is buried in the metal. Additionally, with a shallow shoulder angle in particular, there is no clear ledge for your measuring tool to meet, so it is difficult to get an exact measurement if you want one for a minimum chamber. That is why we use headspace gauges.

The second drawback is that because the cartridges are narrower than the chamber, when a cartridge is seated against the shoulder of a chamber, its own body/shoulder intersect is slightly forward of the chamber body/shoulder intersect, so it's a different dimension. The datum diameter is identical for both cartridge and chamber, providing a common reference point.

Originally Posted by Machineguntony

What are the consequences if the headspace is wrong or off?

In machine guns, it is as described already.

In a bolt rifle with a closed bolt, the consequences are only to brass life. First, if you closed the bolt on the cartridge, you already know the headspace isn't too short to allow chambering. That's number one. It can, however, be somewhat short and the cartridge squeezed in, which raises pressure a bit, but not beyond SAAMI tolerance for individual cartridges in a test set of ten fired to determine Maximum Average Pressure (MAP)¹.

Number two is too long a headspace. Hatcher''s Notebook details an experiment on 30-06 with a special reamer that let them enlarge the chambers by 0.050" (IIRC) and they kept firing and the case shoulders no doubt blew out too long to resize easily, but beyond a point the extractor claw won't let the cartridge go any further forward, and fi your firing pin reaches that distance, it still fires fine. Oddly, he noted an actual increase in velocity as the cut the chamber incrementally deeper, up to a point. I've never see that explained satisfactorily. You would expect the opposite to happen.

I was at the range one day when a fellow walked up to me holding a case with no neck and asked me if it was normal. A friend had lent him his "308 rifle" to hunt with. He bought 308 Win ammo for it, fired one round, and extracted a case with the neck blown out all the way to the mouth. Quick inspection of the barrel stamping showed that it said "30-06". A 308 Win shoulder exceeds the 30-06 chamber minimum diameter by a little bit, but when you combine the plus 0.002" chamber diameter tolerance with the minus 0.008" cartridge diameter tolerance, you get parts that can just squeeze together. The bullet is the right diameter, so it is expelled, but velocity would be low and I would expect a lot from accuracy. But no damage was done by all that excess headspace. It just ruins the case.

Slightly excess headspace lets cases stretch and thin the pressure ring grows thinner faster. That reduces how many reloads you get before the case heads start to separate.





¹ We are used to looking at that number as a pressure limit for individual cartridges, "the SAAMI maximum", but it is actually for an average peak pressure of ten rounds with a standard deviation of 4% of value, and individual rounds within that average are allowed to vary by another number called the Maximum Extreme Variation (MEV), which is a pressure range. In the worst case (virtually never happens), nine of the ten have pressure peaks a tenth of the MEV below Map, and the tenth cartridge has a MAP nine tenths of the MEV above MAP. That works out, the way the pressure statistics are done, to about 18.3% over MAP (varies slightly with individual rounds due to rounding error) for that one cartridge. It is still below minimum proof load levels. CIP allows a hard limit of +15% for individual rounds in an average to go above their MAP.