Head separations cause flat primers?

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edwardware

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I've noticed (again today) that a case that experiences an incipient or partial head separation will frequently show a much flatter or flowed primer than the same load in un-separated brass.

Today I reached the end-of-life of a batch of .223 LC brass after 30+ reloads and 10+ annealings. The two cases with incipient separations show extraordinary primer flow out into the pocket radius, but the rest of the primers are normal for this mid-range recipe. I recall noting this years ago in a couple different rifles too.

Anyone else see this? Any idea why?

. . . and before someone says that head separations increase bolt thrust. . . I agree. Now please explain how the primers are flattened by bolt thrust instead of internal pressure. I thought of that, and I don't think it's mechanically possible; the primer seems to be flattened or flowed by internal pressure.
 
I suspect primers can flow because of inertia against the bolt face. I wouldn't think of it as the bolt smashing the primer, but the primer driving itself into the bolt face because of the combined forces of pressure and inertia due to the case and primer being accelerated where there is excessive headspace. The primer pocket is decelerated by the bolt face, but the primer continues to flow into the pocket radius.
 
I suspect primers can flow because of inertia against the bolt face. . . due to the case and primer being accelerated where there is excessive headspace. The primer pocket is decelerated by the bolt face, but the primer continues to flow. . .

Interesting theory. FYI, my target shoulder setback is 0.001", and I achieve that pretty consistently. With 2-3 firings per annealing, perhaps the pin is creating more headspace. All that to say, I wonder how much velocity the head can build in 0.002-0.004", under 55kpsi pressure. Easy to calculate in a frictionless domain.
 
Interesting phenomenon. labnoti's theory sounds reasonable. I have not had any separations so I haven't seen this.
FYI, my target shoulder setback is 0.001",
Perhaps you should revisit however you are measuring this.
 
FYI, my target shoulder setback is 0.001"

Perhaps you should revisit however you are measuring this.

I determine chamber dimension by neck-sizing a couple firing cycles to ensure the brass is reaching max chamber shape, then measuring shoulder position. My XDie is setup to push an annealed shoulder back 0.001".

An unannealed case will spring back 0.001", so I know it's time to anneal when bolt closing requires slightly more effort, usually every 2-3 firings.
 
If you are only moving the shoulder .001 compared to your chamber you should not have separations no matter how many times the cases are fired.
 
Today I reached the end-of-life of a batch of .223 LC brass after 30+ reloads and 10+ annealings. The two cases with incipient separations show extraordinary primer flow out into the pocket radius, but the rest of the primers are normal for this mid-range recipe. I recall noting this years ago in a couple different rifles too.

Another comparatively-simple explanation could be that these particular cartridges were overcharged or otherwise subjected to higher pressures than the others. In that case, the (near-)separation and flow would just be two symptoms of a single root cause. I have no idea whether that is the explanation here, I'm just positing one possible explanation for what you observed.
 
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If you are only moving the shoulder .001 compared to your chamber you should not have separations no matter how many times the cases are fired.

Yeah, that's what I thought. . . but here I are. I'll have to measure the rest of the batch and see if I find any abnormalities.
 
If you are only moving the shoulder .001 compared to your chamber you should not have separations no matter how many times the cases are fired.
That seems right, but think about what is going on here.

In most cases of head separation, I think you would be correct, the brass stretches lengthwise each time it is fired until it finally pulls apart. But the other thing that goes on at the top of the web is that, when the case expands, it expands in diameter to fit the chamber. But, the web does not expand in diameter. So there is a point along the case wall where it stops expanding to fit the chamber. Now think of what happens when the case is resized, it is squeezed back down in diameter, down to the point just above the web. So, there is a point along the case wall where the brass is worked back and forth in a folding type motion, instead of stretching. Just like folding a metal wire back and forth in one spot, eventually it breaks.

At least the is my story, and I sticking to it.
 
That would be breaking from work hardening, and not from thinning out from stretching too much. I still think he needs to revisit his measurements and how he has the sizer set up. I would confirm I am not moving the shoulder more than I think I am before looking at other possible explanations.
 
I still think he needs to revisit his measurements and how he has the sizer set up.
I agree 100%. And is the same brass fired in more than one firearm??

If brass is brittle (other post) , i would look at the chemicals used in the cleaning process.
 
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I still think he needs to revisit his measurements and how he has the sizer set up.
Yup, I will. Should be easy to verify.

IMG_20180725_052615779_HDR.jpg

And yes, this is dedicated single-chamber brass, and my only chemicals are walnut shell, lanolin, and isopropyl.

I have produced similar looking head separations before by over-sizing shoulders. . . the learning experience that taught me to set 0.001" for bolt guns.

In summary, I can accept worn out brass. What's puzzling me is not the head separation, but the correlation between separation and primer flow.
 
Mixed LC brass , created higher pressure then the rest of the batch. The extra pressure found the weak part of the case. High pressure first, then the separations, for primers to look like that.

Just my guess.



after 30+ reloads
Time to send the brass to the retirement home.
 
. . . I've been reloading this batch so long they've become like little brass children :( (sniffle)

Given that analogy I’m probably guilty of child abuse. I’m loading my .308 competition rounds on the hot side and only getting 7-10 reloads on Lapua brass before the primer pockets get too loose. I never get anywhere close to case head separation.
 
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