Trying to measure some just fired 308 and the case mouths are no longer square.  How would be the best way to get a repeatable length both before and after resizing?

Why are you trying to measure OAL b/a resizing?

Measure off the shoulder, not the mouth, with bump gauge if you are trying to set up your sizing die.

What's the purpose of the before and after measurement?

I use a Giraud trimmer, so everything gets run through it; if it needs trimming, it gets trimmed.  If not, it doesn't.  The Giraud references the shoulder, so as long as I size appropriately, the case length will be good.

If you are measuring to see if the case exceeds the SAAMI maximum, then measure from the case head to the maximum dimension of the skewed mouth.

Working on the lube thing again.  Want to try STP, oil and cheap bastard lube to see which minimizes case growth on sizing.  

Giving up on imperial since it's already proven less consistent and harder to use than STP.  Broke down and bought some lanolin, red heet and a spray bottle to make some of that spray lube.

I've already got the cases measured with a headspace comparitor, just trying to figure out a way to do case length.

As an aside, why would they not be square after firing if they were square before firing?  Different wall thickness so the thick side grows more?

I know it's a waste of time as far as finish processing of the cases go.  But my goal for this exercise it to measure case growth caused by resizing, therefore I need to know how long it was before sizing and how long it is after.

With the head space comparitor, all the cases were within a .001"  I don't have an exact count, but I'd guess 75-80% of my sample measured 1.424" with the remainder being 1.425".

I'm thought if I set up my WFT2 to trim the minimum to get the cases square for measuring before sizing, they still might be in decent shape for the 2nd measurement after sizing.  But the case mouth is too large to fit into the trim chamber, so scratch that.

If I had a lathe, I'd probably make a mandrel where the bottom part was around .310" in diameter for a tight fit in the fired neck with the upper part a larger diameter that would butt up against the case mouth and then measure from the bottom of the case to top of the mandrel.  But alas, no lathe.

I was hoping a machinist had a cool way to make the measurement beforehand.

What kind of variance in dimension are you seeing on length before sizing?  In other words, how far from square do you believe the ends of the case mouths are?

I would record the shortest or longest observed measurement, or take an average of the measurements for that case, and then do the same after sizing.

Also, are you using a micrometer or calipers?  If calipers, are you using the square or beveled edges of your caliper jaws?   If you use the square edges, you will get better measurements (I mention this as it came up in another discussion here recently).
 
Just out of curiosity, what is the goal of knowing how far the cases grow?

If you're bump sizing your cases to minimize stretching/working of the brass, it seems to me that whatever case lengthening that occurs is the unavoidable price of getting the case prepped for another firing (i.e., an arbitrary number).

You're missing the point.

Measuring before sizing means nothing. Casings change size when fired and all don't change the same.

The only way to do a correct comparison is to size and trim the brass. Write down the length that you trimmed them (if you need to I usually trim to the recommended trim length) then after you fire them and re-size measure them again.

Then compare this length to the previous length.

If you don't know what length the brass was before it was fired you simply are not doing anything meaningful measuring it.

When you size the casing it should at least return to it's pre-fired length regardless of how much it shortened when fired. Also regardless of what lube was used.

Motor

Paying attention to case length growth is important if you full length size. The amount a casing grows each cycle will give you an idea of it's expected life span.

It's also an indicator of how much the casing is being altered in size when re-sized which in turn can alert you to possible issues with your rifle or in rare occasions your die.

Typically a casing trimmed to the recommended trim length (usually .010" under maximum) should not "need" trimmed the next time it's cycled.

If your casings are needing trimmed, that is growing more than .010" each time they are re-sized you are heading towards early head separation.

It won't surprise me at all when I see a thread with someone asking why they are getting head separations after only 2 or 3 cycles and they never measure their brass because "it's trimmed every time weather it needs it or not" .......

Motor

I do appreciate everyone's help.  But let's focus on they how instead of the why.  I know this is not a normal request, I'm trying to determine if different lubes have different properties and one of the things I've decided to measure is: Is there a sizing growth/variation difference between lubes.  Not something that I would do every time a case is sized, but only for this study.

I'm using the square flat part of the calipers in the middle of the blade.  Not the knife edge on the outside part.

I guess I'd have to go with the max and min.  Which sucks when measuring 60 cases.

Just measure the high spot, size it, and measure the high spot again. That is how much it grew. The high spot location won't change to a different part of the case.

Did you knock the primers out already?

For purposes of determining if a particular case grows by a certain amount, this is doable.  Just measure at the longest point on the case mouth for both before and after values.  Your experiment will be best served by using cases that were all loaded identically and fired through the same chamber.  Range brass won't tell you jack because the cases won't have any relationship to each other.

Without providing specific data, I can safely say that most cases will be the same length or longer after sizing.  How much depends on a number of factors.  The most important factors are how much the body of the case expanded, and how far you have to push the shoulder back.  In other words, the better the fit between the chamber and your finished sizing dimensions, the LESS the case will grow - except for situations where the load was light, or the load otherwise doesn't stress the brass much.  That's why your experiment needs to be based on cases from the same load and fired through the same chamber.

Yes, the primers are out.  I didn't want anything primer related to affect the measurements.  Nothing was over pressure, QL was saying around 54K psi as an estimate.

All the brass is WCC-10 that I bought, full length resized, trimmed and fired in one rifle.  From the effort of initial resizing, they went through a machine gun the first time they were fired.  A Savage 110 with a factory barrel the 2nd time.

You cannot judge sizing/lube consistency with once fired brass that may have come from several different rifles.

Spring back it going to differ too much to draw any worthwhile conclusions. You may need a couple firing sizing cycles to get everything the same.

I have fired some Lapua and Norma brass through my rifle twice now it it still isn't blown out to my chamber yet with 58k psi loads, the 64k psi loads did but not the 58k 

Do you think spring back would be an issue even if the cases were annealed?  Which they were.

Yes, if one case is x.x20" and another is x.x25" ,the odds of both of them coming out of the sizing die at x.x19" on the first try are very slim......no matter what lube you use.(I'm talking about headspace measurement)

They all came out of the Savage chamber within a .001 of each other.  80%ish were 1.624", the rest 1.625".  Technically, they measured 3.624 and 3.625 but I subtracted the 2.000" for the Hornady comparitor.

In this thread STP Lube in the post dated 7/6/2017 8:34:53 PM EDT (anyway to get a post number or link directly to a post?) all cases where I used STP to resize sized to exactly the same length on the comparitor after firing in my Savage no matter where they were first fired.  The PMJ was out of the once fired cases I bought that were machine gun fired and were fired for the first time in my Savage.  I think the LC cases were once fired by me too, but I don't remember.  They weren't out the last batch I bought, I got them sometime last year.

I'm attempting to duplicate this test along with the one I did on length on the 2nd page of the thread at the same time with the same cases to see if my earlier results replicate.

As long as they came out of your rifle that close you should be fine.(assuming same wall thickness within reason)
I was mainly concerned with the first time you sized them before use in your rifle. 
The data from that experience may not be useful if they were all over the place from different chambers.

If you wanted to do something with merit do this
Phase I
Get some virgin brass 100 pcs
Use a given case lube ie Imperial Die Wax.  
No ultrasonic or SS pin cleaning, after firing brush the necks with nylon brush a couple strokes and clean primer pockets out.
Just remove from rifle direct to mtm box

Get a FL sizer Die with both a carbide expander ball and steel expander ball assembly
Prep 50 pcs with the carbide expander ball and detail prep the brass with the brass trimmed to the trim to length
Prep 50 pcs with the standard expander ball and detail prep the brass with the brass trimmed to the trim to length
Then with a known load for your rifle.  Load all 100pcs of brass.
Then shoot all 100rds
Phase II
Now take 5 pcs of each of the carbide and steel expander ball brass and size without the decapping rod assembly and keep separate for measurement
Now take 5 pcs of the carbide expander ball sized virgin brass and size with carbide expander ball decapping rod assembly and set aside for measurement and size the rest of the lot
Now take 5 pcs of the carbide expander ball sized virgin brass and size with carbide expander ball decapping rod assembly and set aside for measurement and size the rest of the lot
Then with the same load as the first firing for this study, load all 85 pcs of brass
Then shoot all 80rds
Phase III
Now take 5 pcs of each of the carbide and steel expander ball brass and size without the decapping rod assembly and keep separate for measurement
Now take 5 pcs of the carbide expander ball sized virgin brass and size with carbide expander ball decapping rod assembly and set aside for measurement and size the rest of the lot
Now take 5 pcs of the carbide expander ball sized virgin brass and size with carbide expander ball decapping rod assembly and set aside for measurement and size the rest of the lot
Then with the same load as the first firing for this study, load all 70 pcs of brass
Then shoot all 60rds

Phase IV
Now take 5 pcs of each of the carbide and steel expander ball brass and size without the decapping rod assembly and keep separate for measurement
Now take 5 pcs of the carbide expander ball sized virgin brass and size with carbide expander ball decapping rod assembly and set aside for measurement and size the rest of the lot
Now take 5 pcs of the carbide expander ball sized virgin brass and size with carbide expander ball decapping rod assembly and set aside for measurement and size the rest of the lot

Now you can track the differences in sizing methods because after two firings in the chamber the brass
Should be fireformed.

What is the OP's real question and real objective?

It seems to me the original question has been answered and yet there is seemingly so much more going on in and behind this thread.

I'm studying different sizing lubes.  

I want to know which lube gives

a felt easier sizing of the case (subjective) because of my shoulder - the main reason this started
more consistent sizing (data) using a Hornady case comparitor as shown here
more consistent case growth (data) from the case head to mouth.  as shown here

The lubes I'm going to test are 100% STP, mobil 1 5-20w synthetic motor oil and cheap bastard lube.  (lanolin and red heet at a 1:12 ratio - or is it 1:10?).  I've discarded imperial wax because it did not perform as well as STP  in my previous tests, so why try again?

I don't worry about it.  

After processing and sizing, I run all of my brass through my Forster case trimmer which is set to a pre-determined length.  If the brass needs to be trimmed, it is trimmed (and the mouth squared in the process) and if it doesn't need to be trimmed, it is not.  If you are crimping the case mouth then you will want to consider pulling any case that doesn't get trimmed from the reloading stream, but otherwise that will take care of any out-of-square issues.

If you are having a lot of out-of-square cases it may be worth looking into the cause.  Is the brass that inconsistent?  Is the chamber not uniform?  Is there a problem with the bullets?  These would all be cause for some concern.

I've been watching this thread...  wondering why it has taken so long to come up with the obvious K.I.S.S. answer...  Thanx hdwhit!  

The OP is trying to conduct an experiment not trim brass. Did you guys read the OP or just the thread title?

Experiment flawed due to problematic materials (according to OP).  Read title and OP - solution given by another participant and applauded by me.  The proposed solution is simple and "works" for the "experiment" - I don't see a problem here.  

Alternative solution 1 - find (untrimmed) "square" mouth brass for "experiment"!

Alternative solution 2 - find a consistent way to measure inconsistent length brass - doable but takes imagination and some engineering...  >>>  spin the case and measure multiple times then record the longest result.  Results would always be "suspect" due to the imperfections in the experimental design.

If I knew the background genesis of the query, I could probably formulate a "better question" or perhaps understand what is the actual goal being sought...  but the OP didn't give us that so we are are all left to guess...

Another thing to consider is they may be stretching unevenly because the lube is not uniformly applied inside the entire neck.  If I were you, I would determine which lube sizes easiest, then either get one of those dies that "eliminates" trimming, or simply determine the most agreeable trim method for you and use it.  

I understand what you are saying about the experiment, but the results are basically useless in a practical sense.  With a good trim system, I would just rather run all the cases through and let the properly adjusted trimmer trim where it needs to, and not where it doesn't.  Measuring to see what needs trimmed is just another step and additional time spent.

To answer your question posted in the title.  You have to take multiple measurements, mark the brass where you measured it, then take the same measurements again at the same spot.  There is no guarantee that the highest point before trimming will be the highest point afterwards, so if you are not keeping track of what the measurement was where, you are not really gathering any pertinent data.

Place the brass in a pair of V-blocks with the case head against a stop.
Place a dial indicator on the edge of the case mouth; oriented to measure axial length (not radial).
Measure and record displacement shown on dial.
Mark measurement location on case mouth.
Spin case in blocks and record the displacement versus angular position on the case (Li).
Measure the case length at that initial measurement location (Lo).

Process the data using this:  CLi = Lo +Li

This will provide fairly high resolution length versus rotation angle (CLi) information to the OP.

The OP has a lot more going on behind the scenes but if neither the simple "trim them all" nor simple "measure the high spot" approaches works for him, the above will provide endless hours of measurements and data processing enjoyment.

Another thing came to mind. 

Get yourself a NECO concentricity gage.  Part of that rig are some internal mandrels with which you can measure the wall thickness variations as you spin the case in the V-blocks.  Collect some of that data and you may be able to correlate wall thickness variations to case length variations.

Just so you know, with a NECO gage you can measure both the case length variations and the wall thickness variations.

Go for it.  Knock yourself out. 

As I said before, it seems there's a lot more to this than the simple question you asked.  Honestly, I'm just trying to help.

I don't disagree but I won't knock him for trying or not doing the way I would.

I won't do the experiment for him so I don't  care to type out the correct way to do it. Most don't want to take the time to do it property so I answered the question with ,"find the high spot, size, and measure again at the high spot".


I use Oneshot so I  have no use for this experiment, it goes on even on all cases so they size the same.

90% of people's sizing varience problems come from brass being inconsistent, the other 10% is probably from uneven lube distribution. Type of lube isn't a big issue.