I have been loading 308 for my OBR using mixed brass using a  Forrester die set in a coax. Using a hornaday oal gauge I have been getting fired brass that measures around 1.626" measuring from the shoulder. I have been resizing them too 1.618". Today I decided to resize some new IMI match brass and my die is bumping it back to 1.615".I understand the diff in the brass from LC to IMI is probably the reason for the diff.
I then decided to see how they feed in the rifle before heading to the range and after I chamber each one the round now measures 1.610". This is telling me that as the bolt is moving forward and actually setting the shoulder back and additional 0.005".
What I would like to know is what effect this would have on accuracy? I know resizing from 1.626". -1.610" will work the brass but other than that what issues would you see occurring? And should I even attempt to resize them down to the 1.610".

Thanks

You are already resizing the brass too much IMO. I would set the die up to resize to 1.621" - 1.622" according to your posted once fired measurement of 1.626".

Resize a half -dozen cases to 1.622" then see if they will fit in your chamber and that the bolt will close without issues. -.004" shoulder set back from the fired dimension is actually quite a bit. It will allow for easy feeding and extraction and not work the brass too much.

You have discovered what everyone finds out eventually, different brands of brass resize differently. It has to do with how soft the brass is, how old it is, how many times it's been fired, how hot were the loads and at least several other factors that don't really matter.

Keep your brass separated by head stamps and adjust the die to get the sizing you need for each brand. Sizing the brass too much will insure premature case head separation and very short brass life.

When posting, please double space between thoughts so your thread is easier to read.

I don't like measuring fired brass especially brass that was fired in a self loading rifle. It can be done and a lot of people do it but what you really want to know is what size is maximum for your chamber so that you can adjust your brass accordingly.

I say the best way to do this is to back your die off then progressively size just until the bolt will close (gently, not slammed close) then use that measurement as your base line.

Your brass is probably being set back because its so small its actually being slammed into the shoulder kind of like a free floating firing pin.

Motor

.016"  will produce case head separations.
 

I don't think 1.618 is too short as thats what I'm measuring  some new LC LR and LC match at. Additionaly I just measured a new box of Fed 150 grn and the are coming in between 1.616 to 1.617.

As with all autoloaders the fired case will stretch as its being extracted from the chamber so I don't consider that to be the actual chamber size.

I knew this but never put 2 & 2 together.. I've always measured fired brass (and new, factory as well) and set my headspace accordingly.  However now I think I'm going to try shutting the gas off so the case stays in the chamber till it cools off.  May give a better case size/headspace reading?

It sounds like you're not measuring something right. If the rifle was setting the shoulder back .005'' you'd have a lot of trouble extracting that round.

Do it the way Motor said and start big and get smaller till they fit. Close the bolt manually or slowly, don't just hit the bolt release and let it slam home. They need to chamber and extract with little to no resistance.

And like Dryflash said, factory rounds always have too much headspace so they will fit any rifle. I've found some brands to be as much as .015'' short. That's a lot ! Size yours to be .003 -.004'' smaller than your chamber to avoid over working the brass.
New brass should already be sized to factory dimensions, which is too short, so, they'll need to be fire-formed for your chamber first, then resized -.003''.

If your measurements are correct you are setting back too much. Even .008'' is double what you need. You'll get 3 firings before having a head separate.

I agree except for the very last sentence.

I've been setting my full length dies to cam over since I started loading in 1985. I didn't start measuring casings until after joining this forum.

The ones I have measured are around .008" short. I never had any head separation problems until way past 3 reloadings.

The one exception was when RCBS accidentally shipped a small base size die in a standard full length set and we didn't notice.

Motor

The OP is using a Co Ax press. Never used one.

The origin of the last sentence was from an experience a friend had.

One day at the range he walked over and handed me about 12 .308 cases but didn't say anything. I looked at them and they all had a crack 3/4 of the way around just above the web area. I just said, ''You're sizing them too much.''

He said it was the 2nd reload and he was sizing .008'' short to make sure they would fit.

He was shooting an M1A so maybe it was just his rifle doing it, I don't know.

Yes the chamber has everything to do with it. If I ever had a chamber that had max or beyond max headspace, which probably occurs more often than we think, then it's likely I would have had issues to.

Since I began checking I haven't found any sizing more than .008" so I have continued to use cam over.

I am thinking about doing the measurements and trying .003" in my VTR-15 to see if I can improve my .685" varmint load group.

Motor

Generally you'll get the longest case life by shooting cases that don't stretch any when fired.  You'll probably need/want at least .001-002 for an auto-loader (just because, for fouling and smutz-detritus).

Benchrest shooters can get incredible life out of their cases (a hundred cases can go the 3,000-5,000 round life of the barrel) by neck-sizing only and occasionally full-sizing once extraction gets a little tough or sticky.

.002 is generally a good compromise for an auto-loader.  More than that and cases stretch and the case walls thin.  .008 is guaranteeing three to four firings (if that many) before risking a case head separation.

Headspacing in your chamber using the inertia of the bolt carrier group ensures a tight fit but you risk tougher extraction.  You're not helping yourself with precision as each slam-fitting is unique (especially with varied brass thickness from range pick-ups and miscellaneous lots).  Proof will be in your performance -- keep good group data.

Nothing wrong with miscellaneous brass as long as you process it to be consistent within the lot.  Annealing will also help some to normalize neck tension across your brass even though each manufacturer may have different case wall thickness and brass hardness.

A coaxial press is considered by many the best for consistently sizing cases and seating bullets true.

The new IMI cases come in at 1.618 consistently. I'm running them in the forester die just enough to straighten the case mouths and provide consistent neck tension.
The five I fired today measure 1.625" but I know that in a semi-auto that really tells me nothing due to the case stretching upon extraction.
I have 1k of this new brass so it will be a bit before i will need to reload them.
When the time comes I'll try sizing them down to 1.621 and see how they feed and go from there.

There's 2 problems with this idea.

1. Why put off solving a problem you have now ? By the time you finish shooting the rest you may forget about it.

2. You'll work up a load based on cases that are short on headspace, then when you resize them where they should be they'll be sitting in the chamber differently and you'll most likely have a POI change so you'll have to re-work the load.

With new brass I always use cheap bullets to fire form the brass and use them for blasting ammo, but once they are fired in MY chamber I can resize them to what I want. That is, if I'm trying to get a load that is accurate. And the way I see it, ''Why bother using those good components if I can't hit anything with them.'' I go for accuracy because I like to hit what I aim at.

Fortunately I don't buy new brass very often so I don't have to go through the fire forming process much, but I just started loading x54R with 400 pcs of new brass. I'm using Berry's plated bullets to fire form the cases and try to work up a decent load but I'm not going to put a lot of effort into trying to get a super accurate load with them till they are all fire formed.

ETA: Extraction doesn't play as big a part in stretching the cases as you are thinking. The pressure has dropped off significantly by the time extraction occurs. Most of us measure cases fired in our guns and use it for sizing, as should you.