A thread on headspace for those who are bored or love math

[USMC22]

Hi Guys,

I've been here to ask questions and lurk so I figured I'd share some data.

A friend of mine recently purchased a Savage 10/110 BA Stealth (https://www.savagearms.com/firearms/model/BAStealth) and I've been helping him work a load up for it.

A case gauge along with trial and error would have got me "A to B" but what fun is that?

I used my Starrett 6" Electronic Calipers (https://www.brownells.com/reloading/measuring-tools/calipers/6-electronic-caliper-prod41332.aspx) along with Forster Products Datum Dial (https://www.brownells.com/reloading.../ammunition-measurement-system-prod55552.aspx) to obtain measurements on fire-formed brass as follows:

Group 1: Hornady Superformance Match .308 Winchester 168 grain A-Max Lot # 3160280 (Primed)


Group 2: Hornady Superformance Match .308 Winchester 168 grain A-Max Lot # 3160280 (Deprimed)


Group 3: Winchester Deer Season XP .308 Winchester 168 grain A-Max Lot # 3170303 (Deprimed)


Group 4: Hornady Black Ammunition .308 Winchester 168 grain A-Max Lot # 3170303 (Deprimed)


Group 1 Measurements:

Case # (20 Total)

1. 1.6315
2. 1.6295
3. 1.6320
4. 1.6310
5. 1.6290
6. 1.6320
7. 1.6290
8. 1.6305
9. 1.6290
10. 1.6295
11. 1.6290
12. 1.6290
13. 1.6300
14. 1.6285
15. 1.6315
16. 1.6290
17. 1.6320
18. 1.6280
19. 1.6285
20. 1.6295

Sum: 32.598
Average: 1.6299
Sample Standard Deviation: 0.0013138933706635
Variance (Sample Standard): 1.7263157894736E-6


Group 2 Measurements:

Case # (20 Total)

1. 1.6280
2. 1.6285
3. 1.6300
4. 1.6290
5. 1.6285
6. 1.6280
7. 1.6305
8. 1.6290
9. 1.6300
10. 1.6285
11. 1.6270
12. 1.6285
13. 1.6275
14. 1.6280
15. 1.6285
16. 1.6295
17. 1.6285
18. 1.6275
19. 1.6295
20. 1.6275

Sum: 32.572
Average: 1.6286
Sample Standard Deviation: 0.00094032469196326
Variance (Sample Standard): 8.842105263158E-7


Group 3 Measurements:

Case # (20 Total)

1. 1.6290
2. 1.6295
3. 1.6280
4. 1.6280
5. 1.6260
6. 1.6280
7. 1.6285
8. 1.6275
9. 1.6275
10. 1.6275
11. 1.6270
12. 1.6270
13. 1.6260
14. 1.6270
15. 1.6270
16. 1.6280
17. 1.6290
18. 1.6280
19. 1.6280
20. 1.6280

Sum: 32.5545
Average: 1.627725
Sample Standard Deviation: 0.00091010410046556
Variance (Sample Standard): 8.2828947368423E-7


Group 4 Measurements:

Case # (20 Total)

1. 1.6280
2. 1.6285
3. 1.6275
4. 1.6270
5. 1.6265
6. 1.6270
7. 1.6275
8. 1.6275
9. 1.6280
10. 1.6285
11. 1.6275
12. 1.6265
13. 1.6270
14. 1.6275
15. 1.6265
16. 1.6265
17. 1.6270
18. 1.6260
19. 1.6265
20. 1.6270

Sum: 32.544
Average: 1.6272
Sample Standard Deviation: 0.00069585237393845
Variance (Sample Standard): 4.8421052631577E-7


Groups 1, 2, 3 and 4*:


Sum: 130.2685
Average: 1.62835625
Sample Standard Deviation: 0.0014146470463769
Variance (Sample Standard): 2.0012262658228E-6

*All 80 measurements included


Analyzing Groups 2, 3 and 4 (Excluding Group 1)*:


Sum: 97.6705
Average: 1.6278416666667
Sample Standard Deviation: 0.0010230051554233
Variance (Sample Standard): 1.0465395480226E-6


*60 measurements included, Group 1 (Non-Deprimed Brass) was excluded

What I found interesting is that Savage advertises a "blueprinted action" and that they set each chamber to SAAMI minimum. If I set my sizing die up to bump the shoulders back 0.002 on these fire formed cases that puts me at 1.626 (SAAMI minimum)...

I know that this is really just picking apart data and that a different set of calipers along with a different datum dial may vary a bit but I was shocked to see it play out like that.

 

[someguy2800]

That seams normal to me. The fired brass will be under slight tension in the chamber so the bolt would close with resistance on a fired round do to it slightly compressing the shoulder of the brass. Setting back. .002" should be just right and let the bolt close with no additional resistance. I'd say the assembler at savage got it right on the money

 

[Reloadron]

What I found interesting is that Savage advertises a "blueprinted action" and that they set each chamber to SAAMI minimum. If I set my sizing die up to bump the shoulders back 0.002 on these fire formed cases that puts me at 1.626 (SAAMI minimum)...

OK, I will start with making a suggestion. Using your existing gauges you really don't know what you have. I will show an example of what I am getting at and we can review the entire process. Starting with a marked up SAAMI drawing of the .308 Winchester and Cartridge



Cartridge gauges which measure cartridge base to a datum point on the case shoulder are pretty common. Hornady for example offers their Hornady Lock-N-Load Headspace Gauge 5 Bushing Set with Comparator which is about the same principal as what you are using. This type of gauge has a major drawback. The case shoulder fits into a bushing and unless that bushing, in the case of .308 Winchester for example, has a dead accurate corner break where it meets the case shoulder you are not getting a true and accurate measurement. Below is an example of what I am getting at using the mentioned Hornady syatem.



The inserted gauge is an actual 1.630" Go headspace gauge and note the reading on the caliper. The only way to know what your gauge is doing is to check it with a known value, in this case a known 1.630 actual headspace gauge. Less doing the, the gauge can still serve as a useful tool. For example I can measure a cartridge, I can then fire that cartridge and measure it again. Now I set my resizing die to reduce the distance from the base of the case head to the shoulder datum by a given amount, for example 0.002". I am only using the gauge as a comparator, comparing before and after results.

The numbers you are seeing don't surprise me based on how these tools work. If you get an actual chamber headspace Go gauge it will be 1.630". YOu can measure it and know exactly where you are. For example I know when using the above gauge I need to add 0.006" to my measurements if I want an actual true number.

Ron

 

[Chuck R.]

For example I can measure a cartridge, I can then fire that cartridge and measure it again. Now I set my resizing die to reduce the distance from the base of the case head to the shoulder datum by a given amount, for example 0.002". I am only using the gauge as a comparator, comparing before and after results.
Ron

This is exactly how I use my gauges, to set my dies for a .002 shoulder bump for a bolt gun. I then use die shims to achieve the offset so I can use the dies for FL sizing or for different guns. Honestly I don't know if having a true measurement of the fired case VS SAAMI specs even matters for my purposes.

Chuck

 

[Reloadron]

Honestly I don't know if having a true measurement of the fired case VS SAAMI specs even matters for my purposes.

My opinion, it really matters not. While actual chamber headspace can be a nice to know, as long as the rifle isn't showing any indications of really excessive headspace I would not worry about it. Cases come out, case head to shoulder length is reduced and life goes on. Long before any of these gauges were available the hand loader did just fine overall. One merit to the gauge type pictured above is when you get a before number and then an after number you can get a relatively good idea of how much a case grew. Does not matter if the numbers are actual, it just provides a comparison.

Ron

 

[MEHavey]

Savage advertises a "blueprinted action" and that they set each chamber to SAAMI minimum.

A NEW TOY (gotta have it!)

That said (and irrespective of what Savage said), what does carefully resizing a fired case until it just baaaarely chambers read on the Brownells system?

Whatever that measurement that turns out to be... that's the headspace of that gun as measured by that system -- and your reference point from that point on using that instrumentation.

 

[MEHavey]

The only way to know what your gauge is doing is to check it with a known value, in this case a known 1.630 actual headspace gauge

I've got two (full span/soup-to-nuts) 30-06 headspace sets -- one Forster, one Clymer. Both measured differently -- and different from each other -- in the Headspace comparator,

Go figure.... (and that's why I just use the comparator for reloading/sizing in everyday op'ns)

 

[MEHavey]

postscriptum: I'm finding I agree w/ Samfire & Reloadron on the two most emotionally-controversial topics in handloading: Headspace & Lubed cases.

I'm going to either find another board, or we're going to have to establish a new topic area covering proper motorcycle engine oils for British Twins.

 

[hdwhit]

USMC22 wrote:
A thread on headspace for those who are bored or love math

I'm both, so please continue.

 

[Blue68f100]

The problem with using once fired is the brass has not fully formed to the chamber. Neck size hand load it a couple of times then take a measurement it will be close to the actual number. This will eliminate the spring back of once fired new brass.

With that said for us to properly size the brass we only need to maintain the fire formed brass. Which usually means pushing the shoulder back 0.001"-0.002" max or a bolt gun, and 0.003" max for a simi-auto. I anneal all my rifle brass as part of my brass prep. This way I never have to adjust my sizing dies once set. My AR's have min spec chambers too. I have no problem using std dies to achieve this. The type of case lube you use can impact the amount of sizing be done. Thick film vs thin film. So if you decided to use a different lube you ma also be required to adj your sizing die.

 

[USMC22]

OK, I will start with making a suggestion. Using your existing gauges you really don't know what you have. I will show an example of what I am getting at and we can review the entire process. Starting with a marked up SAAMI drawing of the .308 Winchester and Cartridge

View attachment 767348

Cartridge gauges which measure cartridge base to a datum point on the case shoulder are pretty common. Hornady for example offers their Hornady Lock-N-Load Headspace Gauge 5 Bushing Set with Comparator which is about the same principal as what you are using. This type of gauge has a major drawback. The case shoulder fits into a bushing and unless that bushing, in the case of .308 Winchester for example, has a dead accurate corner break where it meets the case shoulder you are not getting a true and accurate measurement. Below is an example of what I am getting at using the mentioned Hornady syatem.

View attachment 767349

The inserted gauge is an actual 1.630" Go headspace gauge and note the reading on the caliper. The only way to know what your gauge is doing is to check it with a known value, in this case a known 1.630 actual headspace gauge. Less doing the, the gauge can still serve as a useful tool. For example I can measure a cartridge, I can then fire that cartridge and measure it again. Now I set my resizing die to reduce the distance from the base of the case head to the shoulder datum by a given amount, for example 0.002". I am only using the gauge as a comparator, comparing before and after results.

The numbers you are seeing don't surprise me based on how these tools work. If you get an actual chamber headspace Go gauge it will be 1.630". YOu can measure it and know exactly where you are. For example I know when using the above gauge I need to add 0.006" to my measurements if I want an actual true number.

Ron

Hi Ron,

Very informative and I appreciate the time you took for the write up.

I intend to use the data obtained to set up a sizing die which would eliminate the need to measure a known value and compare them.

I do see the value in this if I wanted to know the rifles actual values, but even still wouldn’t expect to be that accurate with my tools at home. Comparing my findings to the SAAMI specifications was more for entertainment (and I’m not an engineer so the drawings don’t always make sense at first)

Aaron

 

[USMC22]

Whatever that measurement that turns out to be... that's the headspace of that gun as measured by that system -- and your reference point from that point on using that instrumentation.

Completely agree with this statement!

I've got two (full span/soup-to-nuts) 30-06 headspace sets -- one Forster, one Clymer. Both measured differently -- and different from each other -- in the Headspace comparator, Go figure.... (and that's why I just use the comparator for reloading/sizing in everyday op'ns)

Going back to the above statement, and why this is more a relative value for our purposes than a defining limitation if you will. Work with what you got right?

 

[USMC22]

The problem with using once fired is the brass has not fully formed to the chamber. Neck size hand load it a couple of times then take a measurement it will be close to the actual number. This will eliminate the spring back of once fired new brass.

With that said for us to properly size the brass we only need to maintain the fire formed brass. Which usually means pushing the shoulder back 0.001"-0.002" max or a bolt gun, and 0.003" max for a simi-auto. I anneal all my rifle brass as part of my brass prep. This way I never have to adjust my sizing dies once set. My AR's have min spec chambers too. I have no problem using std dies to achieve this. The type of case lube you use can impact the amount of sizing be done. Thick film vs thin film. So if you decided to use a different lube you ma also be required to adj your sizing die.

I understand what you are saying but given the tolerance of headspace and the known flaws with this system (deviation between known values) I am choosing to omit brass spring back and lube from the equation. I know what you are saying and know that neck sized brass will eventually need to be resized but at some point we need to isolate unknowns, accept them and gather data readily available to accomplish our goals; which in my case was to set up sizing dies and (for fun) examine SAAMI charts.

 

[Reloadron]

I've got two (full span/soup-to-nuts) 30-06 headspace sets -- one Forster, one Clymer. Both measured differently -- and different from each other -- in the Headspace comparator,

Go figure.... (and that's why I just use the comparator for reloading/sizing in everyday op'ns)

I played that game. Sort of like having two watches and not knowing the time. Before I retired I took advantage of our lab and gauge crib. I dragged all my gauges into the plant and had the mechanical guys run them against actual known standards including a Zeiss CMM. The only reason I have actual headspace gauges is when we owned the shop I did quite a bit of barrel work. Following retirement I also lost access to a sweet machine shop, damn I miss that shop but I don't miss getting up every morning and driving to the plant.

Ron

 

[someguy2800]

When I put barrels I put on my savages I just full length size a couple preaces of brass, put on in the chamber and screw the barrel on till there is just a touch of drag opening and closing the bolt. Whatever that is is my headspace. I only shoot hand loads so I only care about what comes out of my dies.

 

[Varminterror]

@Reloadron beat me to it - your 1.62xxx is meaningless in terms of determining actual chamber dimension vs. SAAMI standard, it's just a reference point to help you set your sizing dies. You're talking about a 90degree bore in your comparator, against a datum line in an angled shoulder, and measuring an insertable part, NOT measuring the actual chamber itself.

You'd get closer to an actual reading if you can confirm your comparator diameter, then take a cerrosafe chamber casting, measuring it, instead of the insertable brass. But that still relies upon the accuracy to true value for the comparator throat diameter, and the accuracy of your caliper.

Have you checked your calipers against calibration gauge blocks?

Play that game with an overgassed AR-15 or M1 Garand and things get really interesting - you can measure headspaces on fired brass which came out of a rifle which won't even close back into the chamber, so the measured fired-brass really doesn't tell you anything about the chamber itself.

Brass springback, comparator diameter tolerance, reamer tolerance and level of wear, caliper calibration, etc... Way too many things playing into this little test to be making statements about a minimum spec chamber. Like any rifle - match the ammo you produce on your press to the chamber in your rifle, using the same exact tool for the measurements of the reference and the representatives, and you'll be in the hunt as closely as you can be.

 

[USMC22]

@Reloadron beat me to it - your 1.62xxx is meaningless in terms of determining actual chamber dimension vs. SAAMI standard, it's just a reference point to help you set your sizing dies. You're talking about a 90degree bore in your comparator, against a datum line in an angled shoulder, and measuring an insertable part, NOT measuring the actual chamber itself.

You'd get closer to an actual reading if you can confirm your comparator diameter, then take a cerrosafe chamber casting, measuring it, instead of the insertable brass. But that still relies upon the accuracy to true value for the comparator throat diameter, and the accuracy of your caliper.

Have you checked your calipers against calibration gauge blocks?

Play that game with an overgassed AR-15 or M1 Garand and things get really interesting - you can measure headspaces on fired brass which came out of a rifle which won't even close back into the chamber, so the measured fired-brass really doesn't tell you anything about the chamber itself.

Brass springback, comparator diameter tolerance, reamer tolerance and level of wear, caliper calibration, etc... Way too many things playing into this little test to be making statements about a minimum spec chamber. Like any rifle - match the ammo you produce on your press to the chamber in your rifle, using the same exact tool for the measurements of the reference and the representatives, and you'll be in the hunt as closely as you can be.

Again, I see what you and a few other people are saying...

I understand what you are saying but given the tolerance of headspace and the known flaws with this system (deviation between known values) I am choosing to omit brass spring back and lube from the equation... in my case was [only looking] to set up sizing dies and (for fun) examine SAAMI charts.

Just sharing what I found with the tools available to me.