How to accurately measure (not validate) headspace?

[25WSM]

I have never felt the need to know exactly what the number is. If it's within spec it's within . 003 of what it should be. If it passes the guage test it's a correct chamber length.
Shep

 

[257RobertsMan]

it seems using a go gauge and shims is the only way to get an accurate measurement of headspace. Brass is too soft. Primers are too soft. All will move or form to any pressure from the bolt. We are still confusing the term "headspace".

Please correct me if I'm wrong, is this not the same as using a 3-4 fire formed piece of brass that has been necked sized only, partially inserting a spent primer that has some resistance to entering the primer pocket, inserting it into the chamber and slowly closing the bolt? Will this not tell you the maximun lgth from the contact point of the cartridge shoulder to the bolt face? Is that not the max? Any longer and the bolt will not close (same as a no go gauge). Or technically if the brass shoulder is against the front of the chamber and the primer is protruding out of the primer pocket, measure the protrusion of the primer and that measurement is called Headspace? Or am I looking at this wrong?

 

[Mram10us]

Please correct me if I'm wrong, is this not the same as using a 3-4 fire formed piece of brass that has been necked sized only, partially inserting a spent primer that has some resistance to entering the primer pocket, inserting it into the chamber and slowly closing the bolt? Will this not tell you the maximun lgth from the contact point of the cartridge shoulder to the bolt face? Is that not the max? Any longer and the bolt will not close (same as a no go gauge). Or technically if the brass shoulder is against the front of the chamber and the primer is protruding out of the primer pocket, measure the protrusion of the primer and that measurement is called Headspace? Or am I looking at this wrong?

I'm no expert, so please don't take it wrong, but we misuse the term "headspace" a lot. When the OP said he wanted the exact headspace number, I assumed he meant the headspace measurement as defined by saami which is from the base of the cartridge to the datum point, which of course depends on the cartridge. Not trying to be argumentative just trying to get the answer I assumed he wanted.

 

[257RobertsMan]

I'm no expert, so please don't take it wrong, but we misuse the term "headspace" a lot. When the OP said he wanted the exact headspace number, I assumed he meant the headspace measurement as defined by saami which is from the base of the cartridge to the datum point, which of course depends on the cartridge. Not trying to be argumentative just trying to get the answer I assumed he wanted.

No offence, no argument here, none taken and I'm not trying to be argumentative either, just trying to learn correctly so I don't take misinformation and pass it on to others. I may have misinterpreted what the op was wanting. My wife is constantly telling me I don't always pay attention!

 

[Mram10us]

No offence, no argument here, none taken and I'm not trying to be argumentative either, just trying to learn correctly so I don't take misinformation and pass it on to others. I may have misinterpreted what the op was wanting. My wife is constantly telling me I don't always pay attention!

My wife must know your wife

 

[Jeffpatton00]

As the OP, I'll jump back in here. If people are mis-using the term "headspace", let's define it so we're all on the same page. My understanding is that headspace is the distance between the bolt face and the datum line in the chamber's shoulder. A bolt-action rifle's preferred headspace is the length of the case when pressed all the way forward against the chamber's shoulder, plus the distance from the base of the case to the bolt face, generally 0.002" - 0.003". Do we agree on that?

Some have proposed putting shims or tape behind the case until the bolt becomes harder to close, other have suggested closing the bolt on a protruding spent primer, and the cam action of closing the bolt would press the primer into the pocket, and the remaining OAL (from the case shoulder's datum line to the back of the primer) is the total headspace. Do we agree on this?

If any of this is incorrect, please jump in, I want to make sure I understand the geometry.

 

[Mc Fraser]

I found this

Headspace 101: What You Need to Know « Daily Bulletin

Video shows rifle barrel headspace and explains case separation problems caused by excess headspace. Barrel fitting and gunsmithing tips.

bulletin.accurateshooter.com

 

[Mram10us]

Op, check this out

https://saami.org/wp-content/uploads/2019/12/6.5-PRC-Public-Introduction.pdf

On the bottom right you will see an "x" inside a circle next to the words "HEADSPACE DIMENSION". Follow that to the two spots on the bottom pic (top pic is case, bottom is chamber). You'll see the other two spots with the circled x give you the datum size and the min and max saami length for the headspace.

 

[338 dude]

Please correct me if I'm wrong, is this not the same as using a 3-4 fire formed piece of brass that has been necked sized only, partially inserting a spent primer that has some resistance to entering the primer pocket, inserting it into the chamber and slowly closing the bolt? Will this not tell you the maximun lgth from the contact point of the cartridge shoulder to the bolt face? Is that not the max? Any longer and the bolt will not close (same as a no go gauge). Or technically if the brass shoulder is against the front of the chamber and the primer is protruding out of the primer pocket, measure the protrusion of the primer and that measurement is called Headspace? Or am I looking at this wrong?

I think you're correct that is why I posted it I just learned it on this site several days ago but I am un watching this thread it is going on and on many different opinions but basically I agree with exactly what you were saying

 

[257RobertsMan]

I think you're correct that is why I posted it I just learned it on this site several days ago but I am un watching this thread it is going on and on many different opinions but basically I agree with exactly what you were saying

Thanks Bill, I wish I knew the correct answer and believe that the measurement the OP asked for can accurately be found with fire formed cases as stated and go gauges and shims. In my research later I found a published quote that showed and stated that Headspace is actually the "slop" (for a lack of a better term) that exists from a cartridge not extending from it's base to the shoulder of the chamber. In my thinking 0.001+ of that length would be what I'd call a "crush fit". When we bump back the case 0.002-0.003 then that would be our "Headspace" according to that guy. This seems odd to me because as we know, you can resize to different lengths, bump, partially or FL and thus as a result, a different HS with each technique used, even with the same rifle, brass, etc. Heck, even different brands of factory ammo would give a different HS measurement with the same chamber using that line of thinking (and that may be the correct way to view it when the term excessive headspace is used??) However, we haven't even taken into account brass spring back yet when we resize, bump, etc.
For myself, I want a slight crush carefully measured, then that measurement bumped back no more than 0.004-0.003 to have somewhere around 0.003-0.002" (What I call Headspace) after spring back.

Thanks to all for the information and good luck with your endeavors. Out.

 

[338 dude]

Thanks Bill, I wish I knew the correct answer and believe that the measurement the OP asked for can accurately be found with fire formed cases as stated and go gauges and shims. In my research later I found a published quote that showed and stated that Headspace is actually the "slop" (for a lack of a better term) that exists from a cartridge not extending from it's base to the shoulder of the chamber. In my thinking 0.001+ of that length would be what I'd call a "crush fit". When we bump back the case 0.002-0.003 then that would be our "Headspace" according to that guy. This seems odd to me because as we know, you can resize to different lengths, bump, partially or FL and thus as a result, a different HS with each technique used, even with the same rifle, brass, etc. Heck, even different brands of factory ammo would give a different HS measurement with the same chamber using that line of thinking (and that may be the correct way to view it when the term excessive headspace is used??) However, we haven't even taken into account brass spring back yet when we resize, bump, etc.
For myself, I want a slight crush carefully measured, then that measurement bumped back no more than 0.004-0.003 to have somewhere around 0.003-0.002" (What I call Headspace) after spring back.

Thanks to all for the information and good luck with your endeavors. Out.

I agree with what you say it's just a lot of people don't have such gauges I was impressed by the method I spoke of when it was told to me and thought it would be inexpensive simple and straightforward I too want just a little bolt resistance when I close the bolt with that being said I don't think the actual number means as much as everyone says it all boils down to if you're bolt is too tight adjust your die down until you get just slight bolt resistance I like to keep things simple

 

[J E Custom]

In searching the net for articles on how to measure headspace, I'm a little frustrated because most of the articles with that title actually only tell me how to validate that the headspace is within specs by using go/no-go gauges. That's not measurement, that's simply validation, and I want to understand how to measure the distance from the bolt face (with extractor and ejector removed) to the point where the case shoulder abuts the chamber.

One option, and I think JE Custom suggested this recently, is to take fired cases and bump the shoulder back gradually until your bolt closes with slight resistance. I found with once-fired cases that at 1.560" my bolt handle closes with some firmness, but at 1.557, it closes without resistance, so somewhere between the two is the proper CBTO length. My concern is that this isn't a very precise method, and one man's "firm" is another man's "way too firm", it's too subjective for my taste. The range of lengths in this batch ranged from 1.560 down to 1.556, with most in the 58 - 59 lengths.

A (nearly) direct-measurement approach would be to use the RCBS Precision Mic tool, which appears to offer 2 approaches:
1. Measure a piece of fired brass, and if the tool shows a +.003" measurement, you'd add that to the SAAMI spec to get an overall headspace number. A problem with this approach is that I'd get different chamber measurements if I started with a 1.556" case, versus a 1.560" case. So, this seems as subjective as the manual method above, as well as not being a direct measurement, it involves my plus or minus several thousandths measurement, added to a SAAMI number.
2. The tool appears to offer a plastic dummy round. I'm assuming you chamber that and the chambering action compresses the dummy round down to the actual headspace, then I'd directly measure that a number of times and take an average. My concern with this approach is that the plastic dummy round doesn't look like a very precise item, so I'm not sure if it would accurately reflect the actual headspace.

So, for JE Custom and similarly-capable gunsmiths out there, is there a good way to accurately and directly measure headspace? If I knew that my actual headspace in this rifle is 1.561", then I could be certain that setting the shoulder at 1.558" - 1.559" would be a consistently good CBTO, at least until I need to chase the throat erosion, which is some distance down the road. Since precise groups is based on precise and repeatable control over all the loading parameters, it seems like knowing my CBTO should be [headspace - .002"], for example, would be useful. Thanks for your insight.
Jeff

I prefer using a go gauge only with shims because the go gauge is the minimum head space and has no spring back like a case can have. If the bolt closes on the go gauge, add one .001 shim and if the bolt will still close, replace the .001 shim with a .002 shim. If it will now close on the .002 shim and head space gauge you have the gauge dimension and somewhere between .001 and ,002 head space. depending on feel, It would be .0015 head space. (Use a little grease to hold the shim to the bolt face)

Don't force the bolt closed on any gauge and you can feel when/if the bolt head space is accurate. if you can find shim stock with 1/2 thousandths thicknesses you can measure within 5 ten thousandths.

One other thing, Some head space gauges are .0005 different that other manufactures so what ever gauge you have the gauge length and the shims will give you the exact heads pace to a half thousandth of your chamber.

J E CUSTOM

 

[Carryacolt]

Thanks Bill, I wish I knew the correct answer and believe that the measurement the OP asked for can accurately be found with fire formed cases as stated and go gauges and shims. In my research later I found a published quote that showed and stated that Headspace is actually the "slop" (for a lack of a better term) that exists from a cartridge not extending from it's base to the shoulder of the chamber. In my thinking 0.001+ of that length would be what I'd call a "crush fit". When we bump back the case 0.002-0.003 then that would be our "Headspace" according to that guy. This seems odd to me because as we know, you can resize to different lengths, bump, partially or FL and thus as a result, a different HS with each technique used, even with the same rifle, brass, etc. Heck, even different brands of factory ammo would give a different HS measurement with the same chamber using that line of thinking (and that may be the correct way to view it when the term excessive headspace is used??) However, we haven't even taken into account brass spring back yet when we resize, bump, etc.
For myself, I want a slight crush carefully measured, then that measurement bumped back no more than 0.004-0.003 to have somewhere around 0.003-0.002" (What I call Headspace) after spring back.

Thanks to all for the information and good luck with your endeavors. Out.

Headspace is not related to the case. Its the distance between the bolt face and the shoulder datum point when the bolt is completely closed. We size the brass to fit the chamber to that particular dimension. The dimension varies from rifle to rifle.
The headspace on any caliber is set by Sammi standards.

 

[Jeffpatton00]

Op, check this out

https://saami.org/wp-content/uploads/2019/12/6.5-PRC-Public-Introduction.pdf

On the bottom right you will see an "x" inside a circle next to the words "HEADSPACE DIMENSION". Follow that to the two spots on the bottom pic (top pic is case, bottom is chamber). You'll see the other two spots with the circled x give you the datum size and the min and max saami length for the headspace.

Thx, Mram. From this 6.5 PRC drawing, SAAMI says the headspace should be 1.6486" - 1.6586", or a full 100 thousandths of tolerance. Some responders have said it's not necessary to know headspace, just make sure it's within the safe range with go/no-go gauges, but it seems that view misses several points: 1) for precision, we try to load to a specific bullet jump that our rifle likes, 2) when we're sometimes testing the difference between a 0.010" - 0.020" jump, SAAMI's 100 thou tolerance is way too large for our purposes, we need to know the actual chamber dimension, and 3) for custom barrels, the SAAMI spec doesn't relate to custom chambers cut with custom reamers. These are the reasons that I think we need to know our chamber's dimensions, rather than just validating safety with go/no-go gauges. Otherwise, we're working blind instead of working from data, and in the quest for precision, data is king.