Regarding case elongation w/first firing, will base-shoulder always lengthen to match headspace?

[cdherman]

Ok, long story. Started in 2003 when I purchased an old sporterized 6.5x55 swede. I got 100 rds Lapua brass at the time and started developing loads.

I noticed early on, many of my first loadings had flattened primers. I always assumed this was bad of course. I also noticed then already, that the flattened primers were noticeably extended from the base of the cartridge. And this occurred with relatively light loads.

Well, I was staying well below max book loads, and I think those book loads are already low for old M94 mauser actions, so these were light loads. But I was not shooting long range, I found a decent combo and the old girl sort of languished. Loaned out many times to needy friends. About 30 rounds of brass have gone missing (biggest insult you can do me, is borrow my gun and not return the Lapua brass)....

Fast forward. I have decided to mess with the old girl. I am high tech now, a Hornady OAL tool, etc.

I dug out my brass. 3 baggies. One is unfired. These measure 1.776" base to datum using the .375" insert. The second is once fired brass. These ALSO measure 1.776" base to datum. No cartridge elongation. The third baggie, labled 2-3x (?) fired, measures 1.783", or about .007 cartridge elongation.

Here's the question, and its kind of rhetorical since I think I know already.

But can light loads be so light that they do not push the cartridge head back? And perhaps that explains the brass that has been 1x fired that is the same base to datum measure as unfired brass? And might that explain also the strange appearing flattened primers in low pressure loads that I saw years ago?

Finally, I wonder if this also could be evidence of a rough chamber? Something that would "hold" the brass, rather than let it push back? Just musing....

Regardless, I tried to find some other threads there and wherever via google to no avail. Suspect this has already been discussed somewhere, but here is another go round........

 

[Trnelson]

But can light loads be so light that they do not push the cartridge head back? And perhaps that explains the brass that has been 1x fired that is the same base to datum measure as unfired brass? And might that explain also the strange appearing flattened primers in low pressure loads that I saw ......

Quite a bit of details missing here, but I'll try to give you some things to check out. I don't know the all the specifics of your rifle, sizing die or load, so I'll just start with the most obvious and likely explanation and add some other things for your consideration.
Not to be too obvious or in any way condescending so please don't read any of that into the post. You are using quality brass, if your chamber pressure is not sufficient to cause the brass to obturate, it will not do so. Is your load below recommended minimum charge? Is it utilizing a slower powder within the recommended burn rate range? Are you using a light bullet with a slow powder? Is the fired case blackened with carbon beyond the midpoint of the neck or onto the shoulder? What is the level of throat erosion in the barrel?
As to the flattened primers you have experienced with low pressure loads, there are multiple variables that can contribute to what you have observed and the cause can be pinned down with a little effort. It could be any of the things listed above and additionally, it could be that your chamber is cut to the larger side of specification or even larger than specification. Have you checked the headspace? A chamber casting will show you if it is rough or damaged in any way. Combine that with a sizing die that could be cut to the tighter side of specifications or a 'thin' shell holder and there you have it. It could also be due to bolt thrust from excessive lube on the case or in the chamber. Combine any of those varibles with a load that is not sufficient to obturate the brass and you will see some crazy primer signs, pierced or even blown primers and oddly deformed or caved in brass in extreme instances. Hope this helps. Good luck, have fun, and be safe.

 

[gohring3006]

If you are setting your die up with once fired brass, then you risk over sizing your cases.
I never set up for shoulder setback until the third firing.

 

[dsculley]

When I ordered my die for my 6.5x47 they wanted me provide 3 cases that had been fired 3 times each to ensure that the cases were fire formed to the chamber.

 

[Mikecr]

But can light loads be so light that they do not push the cartridge head back? And perhaps that explains the brass that has been 1x fired that is the same base to datum measure as unfired brass? And might that explain also the strange appearing flattened primers in low pressure loads that I saw years ago?

Yes. With low pressure loads the primer snaps back to the bolt face but the case does not fully stretch back to the bolt face(reseating the primers).

Finally, I wonder if this also could be evidence of a rough chamber? Something that would "hold" the brass, rather than let it push back?

A rough chamber could contribute. I know it's never recommended, but some folks leave a little oil/wax on cases they fire form. This increases bolt thrust though, and you would not want to use a really high pressure load with this plan.

I think you understand results well, and that it's not a problem. Pretty common for cases to take ~3 firings to fully form. Possibly more if Fl sizing between.

 

[Rich Coyle]

Are you guys suggesting three shots with neck sized brass only?

 

[gohring3006]

Are you guys suggesting three shots with neck sized brass only?

Yes, I only neck size for the first three firings.

 

[tim_w]

I would not give any zpecific number but let your brass chamber and load pressure make that determination.

For a case to stretch some part of the case below the shoulder must go past its yield point of the brass and its hardness thickness. Wssm are well known for not fully expanding as they are very thick and hard unless the upper end of the pressure range is used. If I honestly had to guess looking at the cases they are being made out of wsm cases rather than from scratch from a plung.

Yes a rough chamber can effect things but usually it leads to what looks like blown but as been mentioned are unseated primers.

If the bolt closes as easy on a once fired neck sized only case as it does on a fl or factory then I see no reason to fl or bump the shoulder back. Ideally until you feel resistance or it measures larger than chamber size I would neck size only but check runout to see if the chamber is straight and square with bolt face.

Unlike many people I do not see case brass as something that should hold back rearward pressure i.e. bolt thrust. The bolt is designed or should be to do its job with the case pressure any round and a good bit over it without failure. Sure maybe a shortened duty life but it will be many barrels later in my experunce if you keep chamber psi close to saami maxs.

I want my chamber walls slick and let the bolt do its job and the brass contain the pressure not serve double duty trying to add a few psi of thrust off the bolt. Consider all the nitrided barrels. Those chambers are much slicker than even the most polished SS chamber.

Even in my ARs and its ammo, not meant for combat defense whatever catch phase is in vogue, I size my cases to have the very smallest headspace possible. It would only be an issue if I were shooting high vol without cleaning or in a manner consistent with combat.

Ideally for max case life and a be fit to accurcay a chamber and dies should come as a matching set. Die made from a reamer that had its tolernaces set based on the chamber reamer. Sizing the dimensions only the min to allow for reliable chambering.

The sad thing but necessary for the masses is chambers are usually made near the max saami chamber specs and cases are sized to actually below the min spec for cartidges. As to ensure any round in spec will chamber in any chamber that is in spec you do not have overlap meaing even a chamber made to the minimum saami chamber spec will still fit a cartige made to the maximum saami cartidge spec. That leaves an insainlly large amount of slop when you hit those extremes.

For our ARs as we matched bolts we always cut to the minimum for chamber headspace and ran our own tighter tolernaces where it had minimal e:ect on extreme reliablity. One reason is many shoot factory and every factory ammo we checked over the hears were all below or just at the min saami ammo spec dimensions.

 

[cdherman]

Thanks for all the replies. The bit about using necked sized only, 3x fired brass for making custom dies seems to support the notion that not all brass is fully pushed back to the bolt face with the first firing. Guess I did not know that before.

My multiple fired cases are in the realm of .007" longer to the datum, than the unfired and 1x fired cases. So I cannot believe that I have excessive headspace.

As for the loads, they are book loads, in the lower part of the range, as that was where the accuracy was at. They are 120gr spire points, old Sierra bullets that I had laying around at the time. They are chronying at around 2100 fls, so really slow. I had an epiphany just now -- I need to plug that load into quickload and see just what the model says about pressure. Might be enlightening. I have quickload, but I'm not home to look at my reloading records.....

Back to the primers, so I know there are many causes for over pressure flattening/protrusion of primers. But in this case, over pressure was pretty much impossible and there was a visible protrusion of the primer from the base on specifically the 1x fired brass that was not enlongated.

Finally, I looked up definitions of the word "obturate". Seems its really more applicable to the expansion of the thin case walls outward during firing, sealing the chamber. I did not see it being used for the rearward expansion of the case head to the bolt face. Is there a proper term for the rearward elongation of the case during firing? At what PSI should it occur?

 

[Mikecr]

I call it stretch

 

[Trnelson]

Finally, I looked up definitions of the word "obturate". Seems its really more applicable to the expansion of the thin case walls outward during firing, sealing the chamber. I did not see it being used for the rearward expansion of the case head to the bolt face. Is there a proper term for the rearward elongation of the case during firing? At what PSI should it occur?[/QUOTE]

In order for brass to stretch it must first obturate to the chamber wall. If you do not have sufficient pressure to cause the case to obturate it will exhibit minimal elongation.

 

[bigedp51]

"Regarding case elongation w/first firing, will base-shoulder always lengthen to match headspace?"

My 30-30 Winchester at 38,000 cup or 42,000 psi always has the primers protruding. Meaning the case does not stretch or lengthen to meet the bolt face.

Chamber pressure and the cases inside diameter/surface area determines at what pressure the brass will begin to stretch to meet the bolt face.

This is where making a workup load starting at the suggested start load and working up will tell you a great deal. When the primers are flush with the base of the case, you have reached the point of maximum case stretch.

The amount of head clearance with a new case and the chamber pressure will determine if the case will stretch beyond its elastic limits.

You can start a used spent primer and start the primer with just your fingers into the primer pocket. And then chamber this case and let the bolt face seat the primer. The amount the primer is protruding will be your head clearance and how far the case can stretch.

On a fired case and full length resizing you can understand why your shoulder for a bolt action should be .001 to .002. And your shoulder bump will be your approximate head clearance of your sized case.

Below a British .303 case is stretching when fired at maximum military headspace. And at max headspace you can have as much as .016 head clearance.

 

[cdherman]

I call it stretch

Thanks, that was the word I've been looking for.