Help me understand "Head Space"
- Thread starter Prof Young
- Start date
HeadSpace Dimension is HeadSpace Dimension... be it chamber or case; be it base-to-shoulder datum diameter, base-to-mouth, or base-to-rim- front(thickness).
When...
- HSDchamber = Headspacecase, you have no HeadSpace Clearance
- HSDchamber less than Headspacecase, you have negative HeadSpace Clearance (jammed case)
- HSDchamber greater than Headspacecase, you have positive HeadSpace Clearance (rattle room)
When...
- HSDchamber = Headspacecase, you have no HeadSpace Clearance
- HSDchamber less than Headspacecase, you have negative HeadSpace Clearance (jammed case)
- HSDchamber greater than Headspacecase, you have positive HeadSpace Clearance (rattle room)
Neither of the two quoted definitions of headspace is correct. They define what is called head clearance (see SAAMI glossary). Head clearance is the amount by which chamber length is greater than the cartridge requires to fit into it.
Head Clearance = Headspace – Space Occupied by Cartridge Case
When you over-resize a case, you shorten the portion of the headspace occupied by the cartridge, thereby increasing the head clearance. But the headspace is cut into the chamber, so it remains the same.
In the SAAMI system, measurement of the case seat location is defined by the headspace datum. In the CIP and NATO systems, it is defined by the location of the shoulder and body intersect for rimless bottleneck cartridges, same as we did it here until SAAMI came up with the datum approach (sometime after WWII, I infer, from the way the dimensions are defined in publications). The advantage of the datum method is the datum has the same size and location on both chambers and their cartridge cases. However, that is also what has lead to the confusion.
The space in the chamber that is occupied by the cartridge case in the above equation has commonly come to be called "case headspace" or "cartridge headspace" but they are not in SAAMI's glossary as proper terms. I am convinced they came about because of the RCBS Precision Mic, and Stoney Point case comparators. These tools measure a fireformed rimless bottleneck rifle case as an indirect or transfer measurement of the chamber's headspace, and their instructions correctly call that a headspace measurement, even though it's not direct nor as precise as a direct measurement (owing to brass springback). But people seemed to get confused by being asked to measure a case to find headspace, and pretty soon many started to think any case measurement from head to datum, fireformed or not, was the headspace. They were wrong. But the misapplied term has stuck in one way or another, as the two faulty definitions I quoted at the start of this post illustrate. The only thing I know to do until a better term comes along and since the misuse won't seem to come out of the vernacular, is to be careful to modify it by putting the word "case" or "cartridge" ahead of it as mentioned at the start of this paragraph.
In past posts I've called for creation of a term for this. The obvious candidate is headsize. It parallels headspace, but has the same intrinsic confusion caused by the fact the head isn't the only thing included, except in rimfire cases. Casespace is another candidate, but still can be confused as meaning headspace. Suggestions would be welcome if someone has a better idea.
I make every attempt to avoid the term 'head space'. To some everything has head space and every gage is a head space gage. Motive: google search and the nature of a forum. Forums have a tendency to polarize. They just can not help it 'Go Jimmy'.
If the tool is a comparator, call it a comparator, if it is a case gage, call it a case gage. A reloader with little to no ambition can call a Wilson Case gage a drop-in gage, a reloader with shop skills can use the Wilson case gage for what it is, a very precision datum based tool.
Then there is that old saying, it goes something like speaking where the book speaks and being silent where the book is silent.
F. Guffey
sirgilligan
New member
Well, optimistically I think we are all doomed. 
When I read my reloading manuals, which we are all supposed to do, and both use the same description, and then when I read the manuals for reloading dies and headspace gauges and they use the same description, and then I read the online "tips from pros" on the reloading company's websites that use the same description I think the damage is done.
Therefore the discussion, which is true for most discussions, comes down to semantics.
Therefore, if someone asks a question about headspace, context is everything.
Are you re-barrelling your rifle and want to know how to set it so that an in-spec cartridge will go in and allow the bolt to close?
Do you have a new rifle and the bolt will not close on any factory load and you want to know what to measure to see what is wrong or out of spec?
Are you reloading and your cartridges seem to be too long because the bolt will not close, but you measured the overall all length and the cartridge is not too long?
Are you reloading and your cartridges go in fine, but the case head is getting thin only after a few reloads or you are seeing actual case head separation?
When I read my reloading manuals, which we are all supposed to do, and both use the same description, and then when I read the manuals for reloading dies and headspace gauges and they use the same description, and then I read the online "tips from pros" on the reloading company's websites that use the same description I think the damage is done.
Therefore the discussion, which is true for most discussions, comes down to semantics.
Therefore, if someone asks a question about headspace, context is everything.
Are you re-barrelling your rifle and want to know how to set it so that an in-spec cartridge will go in and allow the bolt to close?
Do you have a new rifle and the bolt will not close on any factory load and you want to know what to measure to see what is wrong or out of spec?
Are you reloading and your cartridges seem to be too long because the bolt will not close, but you measured the overall all length and the cartridge is not too long?
Are you reloading and your cartridges go in fine, but the case head is getting thin only after a few reloads or you are seeing actual case head separation?
Unclenick, I've got this solved.
The term that should be used for this measurement is 'Casefit'.
As in, 'for optimum accuracy you should make sure your Casefit is two thousands shorter than the Headspace'
And with the implied nouns added:
'for optimum accuracy you should make sure the Casefit of your case is two thousands shorter than the Headspace of your chamber'
There, world peace
The term that should be used for this measurement is 'Casefit'.
As in, 'for optimum accuracy you should make sure your Casefit is two thousands shorter than the Headspace'
And with the implied nouns added:
'for optimum accuracy you should make sure the Casefit of your case is two thousands shorter than the Headspace of your chamber'
There, world peace
sirgilligan
New member
Doomed I say, doooomed.
Okay, I will do some quick, of the top of my head, brain storming... anything goes...
Cap Gap
Case Spase
Brass ... well you know what rhymes with it
Slop stop
When the chamber is the topic and the thing that is being investigated and/or adjusted then the term chamber could be used.
When the cartridge is the topic and the thing that is being investigated and/or adjusted then the term cartridge could be use.
I don't know, I am too new at all of this, and besides, as a programmer, I get sick of coming up with variable names.
Okay, I will do some quick, of the top of my head, brain storming... anything goes...
Cap Gap
Case Spase
Brass ... well you know what rhymes with it
Slop stop
When the chamber is the topic and the thing that is being investigated and/or adjusted then the term chamber could be used.
When the cartridge is the topic and the thing that is being investigated and/or adjusted then the term cartridge could be use.
I don't know, I am too new at all of this, and besides, as a programmer, I get sick of coming up with variable names.
There has been many times in history when enough people have referred to something wrongly enough times the new term had become an exceptable definition . That is what happen here . It does not make it correct , It's just excepted . As Unclenick said : the head space is cut into the chamber . Head clearance is the open space around a cartridge when it's chambered . Case/cartridge head space is a measurement taken from somewhere on the case . They are not all head space people just call them all head space
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Prof Young
New member
You can see why I am . . . confused.
Sirgillian:
In response to your earlier post about which "headspace" I am asking about . . . I don't know. From the plethora of answers I've received to my OP I can see that this term is used in several ways and as an earlier post noted "context" is everything. And the host of posts above make it clear why a neophyte like myself might experience some ambiguity about "headspace."
So, "chamber" is the space where the ammunition resides when it is lined up with the barrel, the bolt is closed, and it is ready to be fired. "Headspace" is the size of the chamber in relation to the size of the cartridge. If the cartridge is too big then there is not enough "headspace" and if the cartridge is too small then there is too much "headspace." And the point is that all of these dimensions and how the cartridge fits in the chamber has a real effect on what happens to the bullet when the trigger is pulled. Is that the general idea?
Sirgillian:
In response to your earlier post about which "headspace" I am asking about . . . I don't know. From the plethora of answers I've received to my OP I can see that this term is used in several ways and as an earlier post noted "context" is everything. And the host of posts above make it clear why a neophyte like myself might experience some ambiguity about "headspace."
So, "chamber" is the space where the ammunition resides when it is lined up with the barrel, the bolt is closed, and it is ready to be fired. "Headspace" is the size of the chamber in relation to the size of the cartridge. If the cartridge is too big then there is not enough "headspace" and if the cartridge is too small then there is too much "headspace." And the point is that all of these dimensions and how the cartridge fits in the chamber has a real effect on what happens to the bullet when the trigger is pulled. Is that the general idea?
sirgilligan
New member
Prof Young,
To me it sounds like you are on the right track.
I might rephrase your statement to say if the cartridge is too long or too short along a particular line instead of too big or too small. The only reason is that a case could be too big around / too fat and not fit into the chamber.
To me it sounds like you are on the right track.
I might rephrase your statement to say if the cartridge is too long or too short along a particular line instead of too big or too small. The only reason is that a case could be too big around / too fat and not fit into the chamber.
hmm , you know since we are excepting other definitions for head space . Lets make up some more .
The open space around the body of a case when chambered will be called BODY head space
The open space between the the case shoulder and chamber shoulder when chambered but only if the head is up against the bolt face will be called SHOULDER head space
The space between the case mouth and forward most chamber ridge when chambered will be called CASE MOUTH head space
There I think that should cover all the head spaces
Those that talk about head space have to purchase head space gages. It reminds me of being politically correct. Harry Truman lectured someone on being politically incorrect.
People make gages, for the longest I thought smiths and reloaders believed head space gages ware made on Mars by Martians.
F. Guffey
People make gages, for the longest I thought smiths and reloaders believed head space gages ware made on Mars by Martians.
F. Guffey
What does a head space gage measure or gage ?
Is it the space between the bolt face and the head of a chambered cartridge ?
Does a head space gage , gage if you sized your case to small ?
There must be something they were designed to measure ??? 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.
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
New member
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.
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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"
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"
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sirgilligan
New member
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.
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.
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