How to accurately measure (not validate) headspace?

[Jeffpatton00]

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

 

[MNbogboy]

CBTO is a measurement of cartridge base to where the ogive of the bullet meets the lands.
Where case headspace is the measurement from the bolt face to the shoulder in the chamber.
Both are unique to the rifle but unrelated to each other. For example throat erosion will have no effect on case headspace (exception straight walled cases).
If you have a Remington (or clone) style bolt you can use the "Wheeler method" to find both measurements. In a Savage the same method can be applied if the rear baffle is disassembled.
To find actual case headspace you can also use a "go" gauge with shims and then measure them together with a case comparator. I believe several on here use this method.

Hope JE logs into this one, his advice as well as quite a few others is very sound.

 

[cohunt]

.

 

[Jeffpatton00]

CBTO is a measurement of cartridge base to where the ogive of the bullet meets the lands.
Where case headspace is the measurement from the bolt face to the shoulder in the chamber.
Both are unique to the rifle but unrelated to each other. For example throat erosion will have no effect on case headspace (exception straight walled cases).
If you have a Remington (or clone) style bolt you can use the "Wheeler method" to find both measurements. In a Savage the same method can be applied if the rear baffle is disassembled.
To find actual case headspace you can also use a "go" gauge with shims and then measure them together with a case comparator. I believe several on here use this method.

Hope JE logs into this one, his advice as well as quite a few others is very sound.

You're right, thanks, I mis-spoke re CBTO. I do want to measure headspace, though, so I appreciate any and all suggestions. I'll check out the Wheeler method and see what I can learn.

By the way, your handle Minnesota Bog Boy suggests a lot of hours up to your nose in mud, trying not to get carried away by the prehistoric mosquitos up there. What caliber do you use to combat those mosquitos?
Jeff

 

[MNbogboy]

You're right, thanks, I mis-spoke re CBTO. I do want to measure headspace, though, so I appreciate any and all suggestions. I'll check out the Wheeler method and see what I can learn.

By the way, your handle Minnesota Bog Boy suggests a lot of hours up to your nose in mud, trying not to get carried away by the prehistoric mosquitos up there. What caliber do you use to combat those mosquitos?
Jeff

Can't shoot them....State Bird (alternate for the loon).

 

[Mram10us]

Just to clarify further, headspace is the distance to the datum point. Example: .400" on a 6.5cm or .420" on a 28 Nosler. Dont confuse with neck shoulder junctions.

To answer your question, take the go gauge (min headspace measurement) and check for a snug fit like you mentioned. Add tape or shim stock and that should get you to +/-.001".

 

[25WSM]

You really don't need to know the exact measurement. If the go closes and the no-go doesn't it's within spec. The difference between most guages is .003. For resizing your cases use a comparitor on your fired case and set your die to shorten it by . 002 to . 003 and your brass should fit proper. I prefer .003 that way if some of them spring back more than others there is still clearence in the chamber. If some brass code easy and some have resistance you will get verticle in your groups. When you get throat erosion and need to chase the lands this measurement does not change. Your seating depth will change but not your sized case length. Hope this helps you.
Shep

 

[Jeffpatton00]

You mentioned something interesting, tell me more about how shoulder position affects vertical dispersion. I shot this pic when doing load development on a new 223 Wylde semi-auto build, and saw these strings based only on testing different distances from the lands. These were all new brass, FL sized and at that time I didn't know to control headspace, so the different strings changed only by seating distance, and I've always wondered what could have produced these changes, especially the remarkable shift from vertical to horizontal, just changing the seating distance from .010" to .020", then back to quasi-vertical at .030" and .040".

 

[Jeffpatton00]

You mentioned something interesting, tell me more about how shoulder position affects vertical dispersion. I shot this pic when doing load development on a new 223 Wylde semi-auto build, and saw these strings based only on testing different distances from the lands. These were all new brass, FL sized and at that time I didn't know to control headspace, so the different strings changed only by seating distance, and I've always wondered what could have produced these changes, especially the remarkable shift from vertical to horizontal, just changing the seating distance from .010" to .020", then back to quasi-vertical at .030" and .040".

Here's a more easily-accessible image:

 

[BrentM]

I use a go guage and shims. Close the bolt on a go guage and add until you can't, then back off to where you can just close the bolt with minimal.

You can also cast your chamber, its messy. No thanks. I have never found a major need to get that precise about the headspace. Usually brass fired a couple times is at the limit and I use a guage to measure the brass before and during the sizing process. Usually once formed brass is resized to the desire headspace set back you are good to go and subjected to normal brass inconsistency issues from that point forward.

 

[Mram10us]

What cartridge are you asking about?

 

[338 dude]

You might try a pretty simple and ingenious little method that I learned recently on this site take a spent primer and just start it in a case put the case in your chamber close the bolt when you pull it out measure the amount the primer is sticking up that is your headspace

 

[Mram10us]

You might try a pretty simple and ingenious little method that I learned recently on this site take a spent primer and just start it in a case put the case in your chamber close the bolt when you pull it out measure the amount the primer is sticking up that is your headspace

That is cool. I hadn't thought about that. Thanks

 

[Jeffpatton00]

You might try a pretty simple and ingenious little method that I learned recently on this site take a spent primer and just start it in a case put the case in your chamber close the bolt when you pull it out measure the amount the primer is sticking up that is your headspace

That's a pretty clever suggestion, thanks. And for Mram10us, the picture was of .223 rounds, but right now I'm working on 6.5CM through a Savage 110 Ashbury, 24" barrel.