I have several thousand IMI Match 308 brass I purchased a year or two ago.  I have measured each one with a Hornady comparator and they read around 1.617 inches to the datum line.   I'm trying to keep each case exact so when I reload to a certain bullet depth, I know for certain they are all the same.   If one case is 1.620 and the other is 1.617, then the bullets are seated at different lengths and I don't want that.

After firing the above mentioned cases in my custom 308 rifle, the brass measures 1.6215 in length.

I use a Redding Type S Bushing Full Length sizer die.  Case lube is Imperial Case sizing wax with just a touch of the finger lubing the brass.

Off to resizing.  I start and after the first piece of brass is resized, I measure 1.619, next could be 1.619 or 1.6185, then 1.619, then 1.6175.  I even have a few measuring 1.620.

I'm not sure why the different readings if the brass is the same, all fired from the same rifle and the resizing die hasn't been touched.   Is this normal and just expected?

I guess I can just resize away and then measure each case and drop them in separate piles so when I do reload, they should all load with the same bullet depth (depending on the bullet ogive not being off.)

are you lubing the shoulder area?

I figured more traffic is in this section.  I'll go do that.  Thanks.

Just lightly.  Just a finger press down into the lube and wipe the body, neck, shoulder area.  I know too much lube could cause issues but with the amount I put on, I don't believe that is the case.  Although we are talking thousandths of an inch.

my thoughts exactly, don't lube the shoulder when bumping. I generally leave the neck and shoulder dry when sizing and only lube the body. Try that and see what kind of measurements you get. This should help at least some, be sure to clean out the die as well before you give it another shot. (Pun fully intended)

If you're not annealing, that could easily be contributing to the variations in shoulder bump.

As those cases are sized/fired they'll become work hardened (even with a bushing die, and minor bump to begin with), and they won't necessarily do so with consistency across the batch.

Edit: If this is new brass, disregard.

I would say it is different levels of work hardening and/or case wall thickness. This is much less of an issue with quality brass.

Are you sure they are all coming out of the gun the same?

Forgot to include that I did anneal the brass with my annealeez after the first firing.  This is all new brass fired once through the same rifle.

I'll clean the die and just lube the body to see if that makes a difference.

Do you have a way to measure wall thickness?

...and again are you sure they came out of the gun with ALL the same headspace? Sometimes it takes a couple firings to get all the shoulders blown out the same. If they don't start out the same then they often won't size the same.

No, I do not have a way to measure neck thickness....yet.  Anyone have a preference?  Hornady, Sinclair, Forster?

Positive they all came out of the same rifle.  I've only used this brass in my precision 308 rifle.  They all measure exactly the same before resizing, once resized is when I get the different measurements.

Not sure you need to go buy a tubing mic for this one thing, I have a Mitutoyo tube mic but it isn't really a needed purchase.

If you have a careful touch you can probably use your calipers to see if the wall thicknesses are close(within a .001").

IMI 7.62x51 brass measures and weighs close to identical to Lake City 7.62 brass.

It's thick and not really uniform. Make sure you're loading 2.0 grains less powder just like loading LC.

I would use regular dies with an expander ball, or a bushing that is .004" under the loaded neck size followed by a .305" or .306" mandrel.

Heavy cases like this resist resizing. Allowing the case to sit in the die longer or resizing twice can help.

I use the fired case headspace minus .002" for bolt actions, fired case minus a minimum of .003" up to minus .004" headspace for semi-auto's.

You can measure the loaded round necks to determine thickness and neck tension.

Lubing inside the case necks, right? Are necks all the same diameter after expander ball comes back through?  Does expander ball pull harder on some compared to others

when pulling out of die? Then it will pull your shoulders out a bit. To avoid this necks all have to be the same thickness. Like said above, neck thickness.

Also might try to turn the case a half turn after you pull the case out, and push it up in the sizer again.

I sized a few more yesterday and only lubed the body.  Probably need to size a few more to see if it is helping at all but there still is some small amount of movement or different measurements.   I'll have my Hornady neck concentricity and thickness gauge tomorrow but using a caliper I was getting about .03 differences if I remember correctly.  It was .16 to around .19

I didn't know that I should load these lower.  I've been loading around 43gr Varget as that is a good node for my rifle (max 45)  I've been testing at 44 and 44.1 and have had good results.  Primers look good.  How does one tell, besides primers, if you are too high?  Stuck cases?

Not really lubing the inside of the cases, just a finger over the top with maybe a little lube left on it.  Very easy for case to come out of the die.

I do have some Lapua brass I've shot but I might open up a new box and see if this happens with that brass.  Drat,  I though the "match" for the brass meant it wasn't the same as LC and while not the quality of Lapua, it would be close.   hmm.  Have about 1700 new left.    My SCAR is now eyeing it.  LOL.

Brass isn't IMI 7.62x51 it says IMI 308 Winchester March.  If that makes a difference.

So my buddy, who has noticed the same with FGMM brass with his dies moved from the bushing type dies which I am using to a Lee 308 collet size die with undersized mandrel.  .3055 I believe.  Then he full length resizes and first gets a reading (from his comparator) of 1.618 then turns the case and resizes a few more times turning the case each time.  End result is 1.619 which is what he is looking for.   Might try this method next since my method is making sure the outside of the neck is uniform where his method is making sure each bullet has the same neck tension.   Seems like that would be better, imho.

I haven't tried to resize the case multiple times or for a longer duration, I can try that next.

I'm fine with the variances, just wondered why it could be happening and now I've learned about the differences with neck thicknesses.

My process.

All fired cases decapped, cleaned, then annealed in AMP-1

I size/bump shoulder with Redding S die with expander removed.

Then clean up the neck with mandrel die. Trim, then load.

This process has really tightened up both case dimensions, and groups.

Please clarify what you mean by "clean up" and "mandrel die".  AeroE

Weigh the brass unprimed. I'm guessing 178 to 181 grains and literally identical to M80 Lake City ball brass.

If it's significantly lighter I would be surprised.

That error is about 0.002".  It is easy to calculate the correction but you have to know how wide the flats are on your caliper (requires a second caliper to measure).

Your post got me to thinkin'...  Let me apologize in advance for a long winded, meandering reply...  here goes.

1) I think that your ES of 0.003 is very reasonable.  I routinely get ESs of 0.002" to 0.003" with my setup.  The fact that there is variance is to be expected...  the real question is how much variance is acceptable?  I think that avg ±0.0015 is OK & ±0.0010 is very good (one man's opinion, definitely NOT carved in granite).

2) I use lanolin / alcohol lube (described in this forum) - I spray it into a tupperware with my brass, put the lid on and mix.  Take the lid off to allow to dry for an hour or so - mix occasionally to aid "complete" evaporation of the alcohol.

3) My sizing die is a Hornady match die so it full length sizes and uses neck bushings.  I size in 2 stages.  The first stage uses the largest bushing I have (0.338").  For the second stage, I change bushings and use the bushing that will give the correct OD (0.334" bushing for my Federal cases) for the neck tension that I am shooting for.  I have removed the expander from the die and never use it.

4) I have never turned a neck nor have I ever annealed a case.  I have also never weighed or measured cases or bullets for the purpose of segregating them into groups to be treated differently - used or not used for any particular purpose.  I do my best not to overwork my brass so as to not shorten its useful life.

5) The brass that I am working with these days is Federal (FC 308 WIN) brass which I saved from shooting Federal GMM 168s.  Arguably not the best brass on the planet.  I do have 200 new Lapua cases that I plan to put into use in the not so distant future.

6) I trim, chamfer, and de-burr every case every time I resize them.

7) If you want to have some fun try a few things >>>  1) measure the same case (shoulder) 10 times to see if you get the exact same number all 10 times - the results might surprise you!  If you have never done this before, it will likely cause you to work on your measuring technique!  2) Measure a headspace on an unsized case.  Then put it in your press and "almost" size it - i.e pull your handle down almost all the way but don't bottom it out or cam it over.  Then measure the headspace again and see how it has changed... (As you size a case, the headspace increases until you push it back at the bottom of the stroke.)

8) I reload for my bolt action rifles so my objective for the headspace number on my cases is that it should be the same as or a little (1 or 2 thousandths) less than it was before sizing.  "The same as" is OK because the headspace wasn't too long when it came out of the rifle.  The only reason that you might want the headspace a little less in a bolt gun is to allow smooth functioning in less than pristine conditions...  and semi-autos need/want even more wiggle room (smaller headspace length or longer chambers).

9) I never measure COAL as though it matters - it doesn't (other than to tell me whether my ammo will fit in a given plastic ammo box!).  I always measure CBTO as though it matters because it does.  And, just like your shoulder measurements, I get an ES of 2 to 3 thousandths which is an average ± 1 to 1.5 thousandths.

10) I do my best to maintain consistency and use quality components.  I use only Fed GM210M primers.  I use only Varget as a propellant for my 308 loads.  I use only SMK or Berger bullets.  My current crop of cases is Federal - the next crop (waiting in the wings) will be Lapua.

11) I have a group of 40 cases that I use for load development.  These cases have been fired 9 times so far and have not had any problems... and, in spite of what some (many?) might call less than wonderful technique on my part,  my hand loads produce MOA or a little better groups in MY hands using MY factory rifle (Bergara B-14 HMR) - and I guarantee you I am not the best shooter here...

My points here are:
1) I think that ± 1 to 1.5 thousandths (ES of 2 to 3 thousandths) is a decent tolerance on headspace and CBTO measurements.
2) How much could I reasonably expect to improve the quality/accuracy of my reloads by adding techniques that I don't do (like weighting and segregating cases and/or bullets, annealing, neck turning, etc.)?  1/2 MOA? I think that the limiting factor on my accuracy at this point is ME (MY shooting technique).  I think that working on my technique (practice) will produce more improvement than all the other stuff combined.  As a result, I think of some of the more exotic loading techniques as "probably useful for some elite competitive shooters" that compete for scores with expensive/exotic custom rigs...  but that's not me!  
3) How much more case life can I expect if were to anneal my cases?  I don't know yet but I expect the point of failure of my cases to be primer pockets getting too loose not a problem with my necks cracking...  but, in reality, I will have to wait and see...
4) I try not to overthink the process too much.  It's easy to do.  I pay attention to results.  Any changes in technique are made for a reason and then the results are evaluated critically.

Well, that's my 2 cents for now...  I hope I didn't put you to sleep too quickly!...  

I like #4 on your points section!

I like #4 on your points section!

Sorry, delete please.

By clean up, I mean I don’t expand the neck while sizing. Some people, including myself have seen larger variances in shoulder bump when expanding the neck at the same time as sizing. I have much better results expanding on a down stroke using a mandrel die. And I say clean up rather than expand because I chose a mandrel, then modded it so it really only straightens out case neck. No real expansion takes place. My neck tension being set to my specs buy the neck bushing, not the expander.

I don’t know how you want me to clarify a mandrel die. It’s a mandrel die. Mine is made by Sinclair. While looking for it I found a few other manufacturers, but I liked the Sinclair the best. I did buy a couple different expanders for testing, including one made by another company that has a bigger taper.

Yes, it’s an extra step, but one that has shown real results. Same for annealing every time. Brass is sorted into lots, and AMP 1 profile is the exact same every time. Results are the same every time. I usually only load 200 per batch so spending the extra time isn’t real a big deal. I only perform this kind of brass prep on my PRS 6.5 loads.

I did the same with my .308, but netted almost zero gains, so I load it pretty conventionally. I also get best performance out of cheap Winchester brass so I only compete with new brass that is prepped.

I have Lyman M dies and a Sinclair expander.  I like the Sinclair better but the M die has the flare for cast.   Now that I have a lathe I might start turning my own Sinclair type inserts and add the step.

They may have a different amount of the bullet in the neck but the rounds should have the same Over All Length.

The distance to the rifling should be the same.

Seating dies for bullets do not use the shoulder or case mouth to set cartridge length.

I use the same method, except with Redding comp dies and Forrester seater, I prep brass by de-prime, clean with SS pins,Anneal, bump shoulder, neck size, trim to length,Anneal, Sinclair mandrel, prime, load, seat, check concentricity...this on my 300rum ammo..

Yeah, butt you could end up with different amounts of neck length changing neck tension and bullet accuracy...

First off, thanks everyone as I've learned quite a lot with all of the replies.

What I highlighted in red above.  They should be using the ogive right?  I've used the same brass length, measured on comparator... 1.618.  Seated with the Redding competition die and I get different measurements to the ogive.  Sometimes I get 2.222 and sometimes 2.218.  Sometimes I get 5 with the same exact reading 2.222 (good) and then an outlier like the 2.218.  That plays havoc if I'm trying to load all to the same length, not COAL.  I don't care about COAL.   To alleviate this I load long, measure, then dial in to get the proper number.  Then turn the dial out 5-10 and load the next.

My buddy thinks that having the same case reading, 1.618 means the seated bullet should be the proper position each time and if you use 1.616 then it will shorter and 1.620 will have it longer.   I need to re-read all the replies again as I read most of it during my daily commutes to work.

Until I can get to a longer range then the 100 I can shoot at, I'm just blasting away and I'm fine with that too.  

That was going to be my next comment.
A few thousandths of variation in bullet shank in case neck is likely to show up only at longer range in a well set up gun.

The most common way of dealing with small case length variations is to group the shells and trim then to the shortest length in each group.

Fire one shell from each group at range on a target.
Then fire that many shots on another target using shells from a single group.

Start comparing groups.

Verify that your seater plug in your seating die is deep enough to keep the bullet tip from touching it(so that it is actually contacting the bullet close to the ogive)....Some makers make a VLD style seater to replace the stock seater for the new super long pointed bullets....

What type of press are you using?

How stable is your bench? Heavy? Bolted to the wall?

I trim on a Giraud trimmer after sizing. Trimming will allow for all cases to reduce variation in OAL.

Hornady iron press.  The bench is a NRMA reloading bench I build with probably 800/1000 pounds of bullet heads on the shelf.   Not bolted to the wall but I only have a 3/4" top. Really need another to make it stiffer but I don't want it any taller.  I trim with a Giraud too.

I agree with brickeye (unless we’re misunderstanding you).

The headspace dimension of a case (base to shoulder as measured using a headspace comparator like Hornady’s) does not affect bullet seating depth or the loaded length of the cartridge.

You can have a .01 difference in case headspace and identical COAL dimensions.  The difference will be how much of the bullet is visible outside of the case.

If I’m misunderstanding your problem let me know but that’s how I interpreted it.

My (limited) understanding is you can easily get .003 of a difference in seating depth based on neck tension variation, press operation and bullet ogive to tip variations.

I agree.  While we almost always measure our cases and cartridges with our case head as our baseline zero measurement it is also worth noting that the case shoulder datum to seating depth of the bullet is what normally controls the presentation depth of the bullet in the chamber.   A spring loaded ejector button pushes the case shoulder to the chamber shoulder pretty consistently.  So if the op is fussing with making his  cartridges all the same length by readjusting the seating of each bullet in each cartridge he may in fact be adding a variability instead of eliminating one.

Unless you are bench rest match winning your cartridges will unlikely ever be perfect.  Your numbers will not be dead on all the time.   If you are sizing your cases within a few thousandths headspace you are doing very good.   Same with your seating depth measure off the ogive.   Forget the bullet tip other than for magazine fit.    It is irrelevant and a factory production variable with OTM match bullets.   You can trim them but for most of us you’re wasting your time.

I have been 'stealing' BR techniques (and actions) for varmint shooting for many years now.

My varmint rifle is a right bolt, left port Panda action in a synthetic stock.
No ejector either.

Jewel BR trigger set to less than 2 ounces.

I have multiple barrels for it in two calibers.
.22-250 AI (28 inches long and 1 inch at the muzzle) and 6 mm REM AI (32 inches long and one inch at the muzzle).
All are tight neck and require turned brass.
A 'factory' round will NOT chamber.

Neck wall thickness is 0.0100 in.

I sort brass by neck wall run-out before turning.
Then by weight before turning.

They are as close to perfectly matched as I can make them.

Fired twice to form them out before using on critters.

Mostly ground hogs at longer range from a portable bench on a front rest and rear bag.
The 6 mm easily puts the hurt on them out to 600+ yards.

Many have been shot over enough the sight of a human at 300 yards sends them into their burrows.