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Posted: 8/6/2017 5:08:46 PM EDT
I've been fiddling with setting my .308 Win sizing die, and I got to wondering what GI cases are sized to.  I have 15 rounds of 7.62X51 NATO made by Dominion Arsenal (headstamp DA) in 1962, and 5 made by Winchester Repeating Arms (WRA) in 1961, so I measured them with my comparator.

The SAAMI minimum headspace dimension for the .308 Winchester is 1.627", while NATO's standard allows a minimum of 1.626".  The longest headspace dimension I found among these 20 rounds was 1.625", while the shortest was 1.618".  The 15 DA cases ranged from 1.620-1.625" and the WRA cases were from 1.618-1.620.

Sure, these rounds were probably made with the expectation that they'd go in an M14 or an FAL, but having the shoulder 8/1000th BELOW the NATO minimum sounds crazy.

What does this group size .308 Win cases to when loading for more than one rifle, and why?

And an associated question: Which .308 Winchester case gauges do people prefer, and again, why?  I've seen them offered by L.E. Wilson, Dillon, Sheridan and others.  Aside from the Wilson gauges needing oil for preservation, I'd like to know whether or not there's a functional or precision difference among the various gauges.
Link Posted: 8/6/2017 8:13:38 PM EDT
[#1]
A die and shell holder,  used in a good single stage press,  will automatically  set a safe usable sizing for  many rifles of the same cartridge.  I use RCBS . The press should not flex when FL sizing. 

Factory  ammo,  is at times undersize, with extra head clearance.  

Comparator- measure fired brass, push shoulder back about .003" for each rifle.

New factory ammo will stretch and fire form to the chamber.  More than .010"   is not good, but mostly ok for a 1 time firing.    

In 223, .014" head clearence caused case separations  on the 2nd &  3th firings for me.   Had to buy a few new gauges to find the problem.( Dillon shell plate defect.)
Link Posted: 8/6/2017 8:54:27 PM EDT
[#2]
You do realize there is 2 separate set of dimensions.

One is for chamber size.

The other is for ammo size.

Are you using both?

Motor
Link Posted: 8/6/2017 9:33:59 PM EDT
[#3]
Link Posted: 8/7/2017 10:21:01 AM EDT
[#4]
I believe it's almost impossible to get an accurate headspace reading from brass fired from a M1-A. I resize every case to SAAMI minimum. The same length as a .308 "go-gage". This guarantees it will feed and function in any rifle I own.

M1-A's yank the case from the chamber while it"s still under pressure. The result is that the case measures longer than the actual chamber dimensions. Simply bumping the shoulder back -.003" may not be enough.

I chuck .308 brass fired from M1-A's after the forth firing. No exceptions. If you don't, expect case head separations. Lake City brass will last a very long time if fired strictly from a bolt action. M1-A's are a different animal.
Link Posted: 8/7/2017 10:41:16 AM EDT
[#5]
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Quoted:
I believe it's almost impossible to get an accurate headspace reading from brass fired from a M1-A. I resize every case to SAAMI minimum. The same length as a .308 "go-gage". This guarantees it will feed and function in any rifle I own.

M1-A's yank the case from the chamber while it"s still under pressure. The result is that the case measures longer than the actual chamber dimensions. Simply bumping the shoulder back -.003" may not be enough.

I chuck .308 brass fired from M1-A's after the forth firing. No exceptions. If you don't expect case head separations. Lake City brass will last a very long time if fired strictly from a bolt action. M1-A's are a different animal.
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Good info.  I think the  Springfield match grades came with a paper listing the exact headspace for the rifle? 
Link Posted: 8/7/2017 12:08:25 PM EDT
[#6]
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Quoted:
Good info.  I think the  Springfield match grades came with a paper listing the exact headspace for the rifle? 
View Quote
Mine did over 15 years ago.

Not sure if they still do.
Link Posted: 8/7/2017 12:11:15 PM EDT
[#7]
Off the top of my head I think they usually list the headspace dimension at 1.632". This is entirely dependent on their gages and how well the gunsmith held tolerances. Cutting a chamber for an M14 is more of an art than a science.
Link Posted: 8/7/2017 4:18:48 PM EDT
[#8]
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Quoted:
Don't get a case gauge that does 1 caliber, get the Hornady headspace gauge that does all calibers.

https://www.midwayusa.com/product/479704/hornady-lock-n-load-headspace-gauge-5-bushing-set-with-comparator

$33
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I have the Hornady headspace gauge, but it's a challenge to get consistent readings with it.  I wish it had an anvil for the case head, so I could ensure that the case stayed put at both ends.  That would give me more consistent measurements.

For the record, I did an awful lot of measuring with my Hornady gauge, and despite having the 0.40" bushing solidly installed, and working hard at getting the case head as square as possible, I had some cases who's headspace dimension appeared to actually increase after sizing - sometimes by quite a bit.  I just don't feel that for a case this long, I'm able to properly position the head for accurate measurements.
Link Posted: 8/7/2017 4:20:50 PM EDT
[#9]
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Quoted:
You do realize there is 2 separate set of dimensions.

One is for chamber size.

The other is for ammo size.

Are you using both?

Motor
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I'm using SAAMI's cartridge drawing (the current one) and a copy of the ancient STANAG 2310 for NATO dimensions.  The dimensions I mentioned in the OP are for the cartridge.

Edit to add: SAAMI's cartridge drawing calls for headspace dimensions of "1.634 -.007" or 1.627 (minimum) to 1.634" (maximum).  The SAAMI chamber drawing calls for a headspace dimension of 1.630" (minimum) to 1.640" (maximum).
Link Posted: 8/7/2017 5:22:00 PM EDT
[#10]
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Quoted:

I have the Hornady headspace gauge, but it's a challenge to get consistent readings with it.  I wish it had an anvil for the case head, so I could ensure that the case stayed put at both ends.  That would give me more consistent measurements.

For the record, I did an awful lot of measuring with my Hornady gauge, and despite having the 0.40" bushing solidly installed, and working hard at getting the case head as square as possible, I had some cases who's headspace dimension appeared to actually increase after sizing - sometimes by quite a bit.  I just don't feel that for a case this long, I'm able to properly position the head for accurate measurements.
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This is a problem with your technique(s), not with the gage.

1.  Be sure it is properly mounted.  That includes being properly aligned on the jaws.

2.  Punch the primer before making your initial measurement.

3.  Spin the case back and forth just little to settle the case head on the jaw.

4.  Be sure there aren't any burrs causing erroneous readings.

5.  Be sure your jaws move repeatably and are zeroed (a reference standard helps here).

There's more to it but those came quickly to mind.


P.S. - Yes, I have caused cases to grow during sizing.  It is a die adjustment issue you will need to deal with.
Link Posted: 8/7/2017 5:29:22 PM EDT
[#11]
It is normal to have new, unfired cases shorter than SAAMI minimum spec.  I'm not sure I've ever seen a new, unfired case that was at min spec.  I certainly have never seen one above spec.


You originally asked, "How short is too short?"  My answer is two-fold.

1.  It is too short if the cartridge can move so far into the chamber as to slip off the extractor during semi-auto chambering.

2.  It is too short if you get a case head separation on firing.

This latter bit is as much determined by the temper of the brass (age) as it is determined by the resized dimension.  As the brass gets used, it hardens and its ability to stretch when fired gets reduced.  At some point, when fired, the brass will be stretched beyond its elastic and plastic limits and will fail.
Link Posted: 8/7/2017 5:30:00 PM EDT
[#12]
That is the dimensions I would expect. Factory ammo has to be a one size fits all proposition so I would expect it to be loose.

It has to fit in any in spec chamber including of course the smallest so it's typically loose.

Measuring fired casings is not an exact science. Casings spring back after firing and they don't always spring back the same amount.

I know this first hand because I have fired thousands of neck only sized brass in competition. They don't all fit exactly the same. You can feel the difference when you chamber them the next time.

For you to find short fired casings seems perfectly normal to me. Then when you size them the reduction in diameter pushes the shoulder back up where it belongs.

Motor
Link Posted: 8/7/2017 5:34:29 PM EDT
[#13]
The WILSON gages work just fine.  Give them a squirt of spray lube (perhaps Rem Oil) before re-wrapping them in their plastic bag.  It is a small thing to do to preserve your gages.


Of course, when loading for more than one rifle you have to decide whether to share the ammo or to dedicate a batch of ammo to each rifle.  If you share, the ammo has to fit both.  If you dedicate, you should keep track of which cases go to which rifle.  You already know these things, so I'm wondering why you asked.  Did I miss some aspect of your question?
Link Posted: 8/7/2017 9:08:30 PM EDT
[#14]
Discussion ForumsJump to Quoted PostQuote History
Quoted:


This is a problem with your technique(s), not with the gage.

1.  Be sure it is properly mounted.  That includes being properly aligned on the jaws.

2.  Punch the primer before making your initial measurement.

3.  Spin the case back and forth just little to settle the case head on the jaw.

4.  Be sure there aren't any burrs causing erroneous readings.

5.  Be sure your jaws move repeatably and are zeroed (a reference standard helps here).

There's more to it but those came quickly to mind.


P.S. - Yes, I have caused cases to grow during sizing.  It is a die adjustment issue you will need to deal with.
View Quote
The Hornady comparator and appropriate bushing were properly installed.

All of these cases were deprimed and wet tumbled before any measurements were taken.

Spinning the case on the jaws was helpful, but it did not resolve some issues.

No burs were found on any case, either mouth or rim.

My dial caliper consistently zeroed (both with and without the Hornady comparator installed) exactly as expected.

In short, I used every measurement technique I've learned in technical training (both as student and instructor) and then some, and I still wound up with cases that were difficult for me to get consistent repeat measurements on.  As I'd mentioned earlier, if there were an attachment for the other end of the caliper, it would be easier to be consistent...

Knowing that it is possible for cases to increase in shoulder/headspace length is helpful.  I also measured these cases at the base (actually 0.20" above the base, at the specified point for case diameter measurements), but I was not able to see any correlation between changes in diameter there and changes in shoulder/headspace length.  Obviously there's more tweaking to do on this sizing die...
Link Posted: 8/7/2017 10:59:19 PM EDT
[#15]
There's another thing I've noticed and forgot to mention earlier, if the calipers flex it will interfere with consistency.  I have a caliper that is an inexpensive Chinese make that flexes a bit.  Dropping it did not help, either.

Do you center the bases and inserts over the jaws?  Do you have them centered the same every time?  The jaw should bisect the hole when fully closed.

Do you have a solid caliper?

If you really think it will help, attach another base to the other jaw.  I have several bases, OAL gage, headspace gage,...
Link Posted: 8/8/2017 9:04:19 PM EDT
[#16]
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Quoted:
There's another thing I've noticed and forgot to mention earlier, if the calipers flex it will interfere with consistency.  I have a caliper that is an inexpensive Chinese make that flexes a bit.  Dropping it did not help, either.

Do you center the bases and inserts over the jaws?  Do you have them centered the same every time?  The jaw should bisect the hole when fully closed.

Do you have a solid caliper?

If you really think it will help, attach another base to the other jaw.  I have several bases, OAL gage, headspace gage,...
View Quote
Thanks for the suggestions.  You bring up good points.

My caliper is a Lyman dial caliper, which zeros consistently and gives me consistent readings for a number of items (not true "standards" but at least handy and not subject to variation in dimensions).  There is a possibility that the caliper flexes at times, but I hold it with the beam along the side of my hand, with my thumb on the fixed jaw and my index and middle fingers supporting the moving jaw; I had "proper handling of precision tools" beaten into me a long time ago, so I am both careful and smooth when I close the jaws for a measurement.


The comparator usually gets installed on the movable jaw, and is placed so that it's center is near the root of the ramped/tapered section of the jaw.  That means it moves +/- 0.1" from a previous location when it's reinstalled.  However, if I'm doing "pre/post" measurements, such as the ones I'm talking about in the OP, I don't remove the comparator and put the caliper away with it installed so it doesn't move.

The comparator is not designed to have the jaw perfectly bisect the opening; its center lines up with one face of the attaching slot.  But I place it on the jaw consistently, and it stays put when it's in place.  The insert is retained by a setscrew, and while the insert slides in easily, once the setscrew is tightened, it's not moving.  Any time I change the insert, I make sure it's fully inserted into the comparator base.  In this picture you can see that the slot is not quite centered on the comparator...


While looking for a picture of the comparator, I found that Hornady makes a companion anvil for the other jaw of the caliper, so I'm at least going to get one of those to increase the surface area for the head of the case to rest on.  That should cut down on off-axis errors.
Link Posted: 8/9/2017 12:35:58 AM EDT
[#17]
just my observation about that comparator, I noticed that it is very easy to get +/- 0.004" or more wrong measurement by having a very tiny angle if case is not standing straight in it. So when I put it in I have to spend quite some time on setting the case 90 degree straight... thats why this comparator takes so much time to make measurement
Link Posted: 8/9/2017 12:49:14 AM EDT
[#18]
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Quoted:
The comparator is not designed to have the jaw perfectly bisect the opening; its center lines up with one face of the attaching slot.  But I place it on the jaw consistently, and it stays put when it's in place.  The insert is retained by a setscrew, and while the insert slides in easily, once the setscrew is tightened, it's not moving.  Any time I change the insert, I make sure it's fully inserted into the comparator base.  In this picture you can see that the slot is not quite centered on the comparator...
http://www.brownells.com/userdocs/skus/p_749005148_2.jpg
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I always center mine on the jaw.  If you look at the base, it has two opposing screw holes.  These allow you to center the base on the jaws.  Lock tite the set screw and use the thumb screw for easy, repeated removal/replacement (I use two opposing thumb screws).  I know I check mine before every use.  I don't like angular offsets affecting linear measurements.  This is especially true for the bullet comparator.

I will check mine tomorrow and see if I can get it to be 4 mils off and what it takes to be so far off.  I seriously doubt I can be that far off and have everything aligned.
Link Posted: 8/9/2017 7:30:59 AM EDT
[#19]
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Quoted:
just my observation about that comparator, I noticed that it is very easy to get +/- 0.004" or more wrong measurement by having a very tiny angle if case is not standing straight in it. So when I put it in I have to spend quite some time on setting the case 90 degree straight... thats why this comparator takes so much time to make measurement
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Wiggle and/or rotate the case as you are closing the caliper.  The case will self "square-up".  Having said that, I have discovered that measuring headspace is a less than perfect science.  If I measure any given case 10 times, I can get an ES of 1 to 2 thousandths for the group of measurements.  And is doesn't matter which measuring device I use...  I have the Hornady model as well as the RCBS Precision Mic and reload for 308 Win...  If you think about it, 0.001 to 0.002 top to bottom variance is actually pretty good although I expected (wanted) less than 0.0005!
Link Posted: 8/9/2017 12:19:40 PM EDT
[#20]
Then add to that measurements taken with calipers even by people who use them on a daily basis easily have a one to two thousands margin of error.

That's why many of us call them "very-near calipers"

Motor
Link Posted: 8/9/2017 1:08:41 PM EDT
[#21]
My Grendel Lapua brass was .015-.018 shorter than my chamber(fired brass).

I use the Hornady comparitor and it repeats perfectly, there is no flaw in the design. Pop the case in and rotate while wiggling, the case straightens itself, you can feel if it is flat or not. I can measure a case 100 times and get the exact same reading. 

The only problem would be if your case heads are crooked or uneven.


 
Link Posted: 8/9/2017 1:35:18 PM EDT
[#22]
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Quoted:
...That's why many of us call them "very-near calipers"...

Motor
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I see what you did there...  
Link Posted: 8/9/2017 8:01:40 PM EDT
[#23]
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Quoted:
My Grendel Lapua brass was .015-.018 shorter than my chamber(fired brass).

I use the Hornady comparitor and it repeats perfectly, there is no flaw in the design. Pop the case in and rotate while wiggling, the case straightens itself, you can feel if it is flat or not. I can measure a case 100 times and get the exact same reading. 

The only problem would be if your case heads are crooked or uneven.


 
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For me, I think just adding the anvil to the other jaw will help me feel the difference between "straight" and "almost."  It's worth $12-$15 for me...  

I never seemed to have issues with the neck settling in the comparator, it's just getting the case head to play nice with the other jaw - and since both jaws have a bevel for fine measurements, there's not really a lot of real estate to get the case settled on.
Link Posted: 8/9/2017 8:05:09 PM EDT
[#24]
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Quoted:
Then add to that measurements taken with calipers even by people who use them on a daily basis easily have a one to two thousands margin of error.

That's why many of us call them "very-near calipers"

Motor
View Quote
My technique is to open and close the jaws a bit as I put the case in, and turn the case gently while holding a tiny bit of pressure on the moving jaw.  I take my time to read the dial carefully, and I'm a big fan of the tried-and-true "tilt the dial to read it precisely" technique.  

With that, I still am confident that I'm getting no better than +/- 0.00005 (5/10,000) no matter what I do.  It's not that tiny bit that bugs me, though.  It's the big differences between measurements, especially pre-post measurements, that are hard to fathom.
Link Posted: 8/9/2017 8:12:27 PM EDT
[#25]
All of my rifles came with a case gauge between the bore and the bolt.
Link Posted: 8/9/2017 10:19:36 PM EDT
[#26]
Quoted:

I still am confident that I'm getting no better than +/- 0.00005 (5/10,000) no matter what I do.  

Huh?  Something is wrong with these numbers.  50 micro-in with a caliper!!!???  

The 5/10,000 inch (0.5 mil) is typical range I see once the case is settled in the gage and I spin the case.


It's not that tiny bit that bugs me, though.  It's the big differences between measurements, especially pre-post measurements, that are hard to fathom.

Which pre/post are you referring to - pre firing/post firing or pre-sizing/post sizing or...???
Link Posted: 8/9/2017 10:31:22 PM EDT
[#27]
Discussion ForumsJump to Quoted PostQuote History
Quoted:

My technique is to open and close the jaws a bit as I put the case in, and turn the case gently while holding a tiny bit of pressure on the moving jaw.  I take my time to read the dial carefully, and I'm a big fan of the tried-and-true "tilt the dial to read it precisely" technique.  

With that, I still am confident that I'm getting no better than +/- 0.00005 (5/10,000) no matter what I do.  It's not that tiny bit that bugs me, though.  It's the big differences between measurements, especially pre-post measurements, that are hard to fathom.
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You're not measuring that.
Link Posted: 8/9/2017 10:52:06 PM EDT
[#28]
The Hornady case comparator is just that, it's not a gage.

It may be repeatable, but who says it's accurate?  It's not the same thing.

You need a known dimension master, preferably something that approximate the dimension you are seeking.  A master you can zero on.

Get a headspace gage and master off of that.  Otherwise you're just guessing.
Link Posted: 8/9/2017 11:00:13 PM EDT
[#29]
Link Posted: 8/10/2017 4:59:30 PM EDT
[#30]
Discussion ForumsJump to Quoted PostQuote History
Quoted:
Quoted:

I still am confident that I'm getting no better than +/- 0.00005 (5/10,000) no matter what I do.  

Huh?  Something is wrong with these numbers.  50 micro-in with a caliper!!!???  

The 5/10,000 inch (0.5 mil) is typical range I see once the case is settled in the gage and I spin the case.


It's not that tiny bit that bugs me, though.  It's the big differences between measurements, especially pre-post measurements, that are hard to fathom.

Which pre/post are you referring to - pre firing/post firing or pre-sizing/post sizing or...???
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Half way between the lines on a 1/1,000 dial is 5/10,000. Edit to add: But 1/1,000 is 0.0001, and half of that is 0.00005.  My decimals above have too many zeros...
Link Posted: 8/10/2017 5:00:31 PM EDT
[#31]
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Quoted:
I've been following along, but had nothing to add that Trollslayer didn't cover until now.

The part of my calibers that I measure with (in the middle) is flat and about a 1/16 of an inch wide. The bevel on the jaws on my caliper is just the first 1/4 inch.

Are your caliber jaws beveled the whole way? If so, get a different set that has flat jaws in the middle. 

My measurements don't vary more than .001.

eta,

https://www.ar15.com/forums/t_6_42/480233_AR15-Fundamentals-and-Reloading-Gains.html&page=1&anc=4829858#i4830097

The post at the above link by monkeypunch has a great pic of the same calipers that I have and you can see the flat part of the jaws I was talking about.
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The bevel is only about 1/4", but it's hard to get the comparator to stay put if I keep it that far out.  I center it on the root of the bevel.
Link Posted: 8/10/2017 5:20:15 PM EDT
[#32]
Link Posted: 8/10/2017 7:02:44 PM EDT
[#33]
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Quoted:

Half way between the lines on a 1/1,000 dial is 5/10,000.
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Yes, 5/10,000" = 0.0005" =  500 microinches.  

Your decimal fraction has too many zeroes.  <--Minor technical correction
Link Posted: 8/10/2017 7:04:00 PM EDT
[#34]
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Quoted:
You don't want either the base or the case on the bevel.

Both need to be on a flat surface of the jaws.
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I agree, use the widest part of the jaw, not the beveled portion.
Link Posted: 8/10/2017 11:02:42 PM EDT
[#35]
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Quoted:


Yes, 5/10,000" = 0.0005" =  500 microinches.  

Your decimal fraction has too many zeroes.  <--Minor technical correction
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You're absolutely right.  Reading is fundamental.  I should do it more.
Link Posted: 8/10/2017 11:03:51 PM EDT
[#36]
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Quoted:


I agree, use the widest part of the jaw, not the beveled portion.
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My location was chosen with the idea that the caliper was supposedly "most accurate" at the knife edges of the jaws.  I will see if I can get better consistency by moving the comparator off the bevel entirely.
Link Posted: 8/10/2017 11:16:48 PM EDT
[#37]
Link Posted: 8/10/2017 11:21:16 PM EDT
[#38]
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Quoted:

My location was chosen with the idea that the caliper was supposedly "most accurate" at the knife edges of the jaws.  I will see if I can get better consistency by moving the comparator off the bevel entirely.
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Honestly, I've never heard that one before.  If anything, that is the least accurate place (most susceptible to bending and deflection due to tolerance stack-up).
Link Posted: 8/11/2017 7:59:06 AM EDT
[#39]
Link Posted: 8/11/2017 9:49:29 AM EDT
[#40]
Discussion ForumsJump to Quoted PostQuote History
Quoted:

My location was chosen with the idea that the caliper was supposedly "most accurate" at the knife edges of the jaws.  I will see if I can get better consistency by moving the comparator off the bevel entirely.
View Quote
Put it as close to the dial as possible while still being on the flat of the jaw.The accuracy is no different at the tip, I have no idea where that comes from. If anything it is worse due to flex.

Besides that you don't need accurate measurements you need consistent measurements.
I don't even zero my dial when I put the comparitor on, it is pointless.
 All I care about is what the dial reads, this one was 20 1/2 that one was 21 1/5  that one is a thou longer. That's it, that is what this tool is for, don't care if it is 1.4205" or 10.8205", I am comparing.


And by the way, all 4 of my comparitor bodies had burrs in the slots preventing them from sitting flat on the jaws.


At 22 seconds, that is how you put a comparitor on. Note how I am as far from the tip as possible and how I close the jaws on the comparitor and spin it a bit  between the snug jaws to square it, then tighten it to the jaw.

Link Posted: 8/11/2017 2:14:07 PM EDT
[#41]
Thanks for troubleshooting my technique.  I'm pretty sure that my placement of the comparator has been responsible for this issue, both for the poor consistency I've seen in placing the case head on the jaw, and for flexing the jaws while I was holding them.

I am not certain where I got the idea that the beveled end of the jaws was most accurate.  I can recall using a vernier caliper to explain verniers to other people, but not anything about the jaws...  Ah well.

I can see lots and lots of ways my caliper use has been potentially off in a number of ways, often due to the position of the comparator on the jaws, and due to my thumb pressure on them - mostly to keep things from falling because the case was so far out on the jaw.

I'm out of town this weekend, but when I get home, that's something I'm going to jump on.
Link Posted: 8/11/2017 2:19:36 PM EDT
[#42]
Popinfresh, thanks for the encouraging words.

One thing I have figured out for myself was the possibility of burs, and I've ruled that out.  Not just on the red part of the comparator, but on all the headspace inserts too.

One question: is there supposed to be a setscrew in the side of the comparator base opposite the thumbscrew?  I'm not sure if there is one on mine or not.  If it's supposed to be there, I can certainly use that to tighten up the base on the caliper jaw.
Link Posted: 8/11/2017 2:55:46 PM EDT
[#43]
Link Posted: 8/14/2017 9:28:21 AM EDT
[#44]
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Quoted:


One question: is there supposed to be a setscrew in the side of the comparator base opposite the thumbscrew?  I'm not sure if there is one on mine or not.  If it's supposed to be there, I can certainly use that to tighten up the base on the caliper jaw.
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No, just the little thumb screw to tighten to the jaw.
Link Posted: 8/14/2017 10:13:18 AM EDT
[#45]
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Quoted:
No, just the little thumb screw to tighten to the jaw.
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Excellent.  I thought I'd lost the setscrew.  I figured the hole was all the way through to either facilitate manufacturing, allow you to reverse the base, or both.
Link Posted: 8/16/2017 8:19:59 PM EDT
[#46]
Holy "bad measurement practices" Batman!  Changing where the comparator was on the jaw made a HUGE difference in both the actual number and the consistency of measurements.

I got home from out of town yesterday evening and didn't have it in me to mess around with this, but today I dug in and measured a bunch of brass.  I moved the comparator's base to as close to the beam of the caliper as it would go and secured it, making sure that I had it lined up so that the jaws were centered on the device.

I measured the same 50 cases that I'd found wildly varied headspace values for, and even went back and randomly re-measured 10 of them.  First, all the values were between 1.6250 and 1.6310, with most (25% of them) at 1.6290.  Second, every re-measure was identical to the original value.

Later, I'm going to re-measure the issued NATO ammunition to see if I was as far off with that as I was with everything else.
Link Posted: 8/16/2017 9:12:48 PM EDT
[#47]
+/- 0.003" is still a lot of variation on headspace if the brass is all from the same lot.  


I don't know you sizing die or lubrication but I'd look at non-uniform lube first because it's easiest to diagnose.


Lube the shit out of some cases and size them.  Be sure the lube gets IN the necks to lube the passage of the expander ball. 

Check the uniformity of those.  I'd say +/- 0.001" max allowable deviation or something else needs some work.

I picked that value somewhat arbitrarily but when you are trying to push the shoulder back only 0.002" to 0.003", it seems about the right value.
Link Posted: 8/16/2017 10:53:11 PM EDT
[#48]
Link Posted: 8/17/2017 7:45:41 PM EDT
[#49]
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Quoted:
+/- 0.003" is still a lot of variation on headspace if the brass is all from the same lot.  


I don't know you sizing die or lubrication but I'd look at non-uniform lube first because it's easiest to diagnose.


Lube the shit out of some cases and size them.  Be sure the lube gets IN the necks to lube the passage of the expander ball. 

Check the uniformity of those.  I'd say +/- 0.001" max allowable deviation or something else needs some work.

I picked that value somewhat arbitrarily but when you are trying to push the shoulder back only 0.002" to 0.003", it seems about the right value.
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This brass is not necessarily from the same lot, though most of it has the same year headstamp (LC 78), and it's almost certainly also been fired in M60s, which implies a lot more "working" of the brass than cases fired through a newer M240.  I ran these cases originally to get a sense for how much tweaking my sizing die might need to get the brass sized.  So I'm not even suggesting that I am 100% satisfied that my sizing die is fully adjusted.  

I'm just happy that the same cases that I had measured earlier and gotten bizarre numbers no longer seem as grossly off as they did, just because I'm now measuring them with a better process.

I'll get the die fully adjusted and we'll see how close the results are to each other.  I'm willing to bet that with just a little bit of tweaking I'll get a lot more consistent numbers.
Link Posted: 8/17/2017 8:32:27 PM EDT
[#50]
I had a similar situation once.  I sized them all a little more (smaller) than normal so they would all easily chamber for one firing in my rifle.  After that, I went back to the more normal 0.002" to 0.003" set back during sizing.  That worked for me.

I remember one piece of brass was particularly stiff/springy and required a radically different die setting to get it to the proper size.  I only had to do that the one time.

Now, if you were annealing the cases,...  
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