Cause of case head separation

James Adamson

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After 3 firings I noticed the signs of case head separation. A bright shiny ring just above the case head, couldn't feel anything with a bent paper clip so I opened one up and could just make out the faintest ring on the inside too. What causes head separation other than excessive head space? I full length size but only bump the shoulder back .003"
I haven't had this issue before, am I missing something? Any help gratefully received.
 

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Are you certain you’re only bumping the shoulder 0.003”?
 

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What round and make of brass? The usual culprit is indeed headspace but what matters is how. Either the chamber is long, not insurmountable, or the die is setting the shoulder back too far.
 

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All cases stretch and can separate over time.... just a matter of degree and the overall case life depends on which type of fatal degradation comes first type thing.

Three things usually in concert. One like you posted is excess headspace causes more rapid stretching thus more rapid path to separation. Another is pressure. At a certain pressure, brass begins to flow. It is getting pressed outward against the chamber walls and really the front part of the case, forward of the web, has only one direction to go... forward thus the stretching. The most common area to show that stretching and thinning is the junction of the web and body (the location of the shiny ring) and most common location of separation. One factor sometimes left out of the equation is cartridge design. Seems simple but fairly hard to describe and to some extent a matter of degree. Some designs tend to stretch more than others. Some of reason has to do with amount of headspace control in the design.

Another part of the equation is type and quality of brass... but best to leave that out of this discussion and usually not as great a factor as certainly the first two variables of design, headspace and pressure.

Interesting also is that in response to the necessary evil of some headspace for function, the case stretches in two directions. The sequence is usually lost in translation or in a form I often hear as misunderstood mechanics. The cartridge is sitting in the chamber. The headspace control is holding it from moving forward...(+/- the built-in sizing headspace required for basic rifle cycling function).... mostly but not completely. The firing pin strikes the primer. This is where the misunderstood mechanics begin. The firing pin simply does not have the impetus force available compared to the inertial mass of the cartridge plus the headspace control of the case to cause much if any excess or extra headspace issue. The primer on the other hand does have the energy to overcome that inertia mass of the cartridge and enough to over-run the headspace control of the cartridge. A case for example with a large, sharp shoulder may have enough headspace control to mitigate most of the primer force and the over-run will be minimal. Continuing on with the sequence of firing. The primer fires. The primer "wants" to back out of the case. Depending on mass of cartridge, charge in primer, type and amount of headspace control... the primer backs out against the bolt face simultaneous to the case moving forward to the headspace control and over-runs that control to some degree. The charge lights and comes up toward full pressure. Momentarily the front part of the case sticks to the chamber walls under the intense pressure. At the same time the head of the case is driven back against the bolt face, reseating the primer and stretching/thinning the case wall just forward of the web. The case depressurizes as the bullet leaves the muzzle and the spring-back in the brass allows the case to ever-so-slightly shrink back from the chamber walls and be extracted. With a normal load, the pressure was not excessive, the primer "looks" normal but the "hidden" stretching/thinning has been done.

If in doubt about how this sequence occurs, load a cartridge to relatively low pressure- maybe on the order of 25-30 kpsi. Seat the bullet deep enough to stay well clear of the lands. Clean all lube off the outside of the case and from within the chamber. Fire the round. If the pressure is enough to momentarily stick the upper case to the chamber walls but lower than required to stretch the case rearward to contact stop against the bolt face, the cumulative amount of headspace will show as a backed-out primer. That backed-out primer was not caused by firing pin impact force! :)

So, for example, your cases are sized for .003" headspace in your rifle and your case design allows an additional .oo3+ upon firing. The net cumulative headspace in that instance would be .006". Add to that the pressure component and it all leads to more stretching per shot thus shorter case life and fewer shots until case separation compared to a case that has a more positive headspace control design and is loaded at lower pressure which will stretch at a lesser rate per reload/shot.
 
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Ray B

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In short, because the cartridge must have some clearance so that the bolt will close, there is space in the chamber. On firing the case fills out that space. the more space there is the fewer firings are needed to cause the brass ahead of the web to thin to the point of separation. Additionally, higher pressure loads can accelerate the process. case expansion will happen with all cases so the goal is to limit it to the extent that the case "wears-out" from some other cause such at splitting at the neck.
 

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JME, If you resize and reload the case multiple times, eventually the brass has stretched enough near the web that it will separate. That said, I have never had this issue with only 3 reloads. Usually in the 6+ reloads.
As mentioned headspace, but my experience is the very first round out of a 1991 Argentine Mauser notorious for headspace issues.
Too high pressure loads. Trying to get the last fps out of a load.

Best of luck figuring this out! Not a good situation!
 
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James Adamson

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Thankyou all for your replies so far.
@fourfive8 very good information.

I am shooting a tikka T3x in 30-06. I have 3 different loads worked up for this rifle, a cheap practice/plinking load using winchester brass and 2 "good" loads using lapua brass. I measured and remeasured the brass again last night and all came out .003 to .004 of shoulder bump. After looking at it again this morning I am not sure if it is case head separation or just a mark left by the die. Here is a couple of photos, let me know what you think.

20200423_091957.jpg
20200423_092014.jpg
 

Wyatt Smith

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I bought a No. 4 MK1 Lee Enfield and fires the first shell pulled back the bolt, and the case head came out, the rest stayed in. Now I check that stuff.
 

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Thankyou all for your replies so far.
@fourfive8 very good information.

I am shooting a tikka T3x in 30-06. I have 3 different loads worked up for this rifle, a cheap practice/plinking load using winchester brass and 2 "good" loads using lapua brass. I measured and remeasured the brass again last night and all came out .003 to .004 of shoulder bump. After looking at it again this morning I am not sure if it is case head separation or just a mark left by the die. Here is a couple of photos, let me know what you think.

View attachment 343897 View attachment 343899

Hello James.

Thanks for sharing the image. To my eyes, that could simply be the witness mark from where the die stopped 'squashing back' the walls of the case as opposed to incipient case head seperation?

You get this sometimes when sing a FL sizing die to just 'bump' the shoulder a bit as it effectively doesn't squash back the base of the brass when you size, leaving the base and that mark at the point when the sizing operation stopped.

When I've seen the case head seperation phenomenon, it's always been a thinner, more clearly defined mark and a more noticeable thinning of the brass on the inside, so that it can be felt with the paper clip trick. Also slightly higher up the case.

3x firings is pretty poor for 30-06 if you're not pushing the loads hard.

With my .270 I tend to bump back the shoulders 2 thou and I expect 7-10 loadings out of my relatively mild 'plinking' loads and 5 out of the fairly hot hunting ones. I also usually find that the necks split before I get this issue.

There are plenty of others on here with more experience of course, but that's my opinion!

Al.
 
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James Adamson

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Hello James.

Thanks for sharing the image. To my eyes, that could simply be the witness mark from where the die stopped 'squashing back' the walls of the case as opposed to incipient case head seperation?

You get this sometimes when sing a FL sizing die to just 'bump' the shoulder a bit as it effectively doesn't squash back the base of the brass when you size, leaving the base and that mark at the point when the sizing operation stopped.

When I've seen the case head seperation phenomenon, it's always been a thinner, more clearly defined mark and a more noticeable thinning of the brass on the inside, so that it can be felt with the paper clip trick. Also slightly higher up the case.

3x firings is pretty poor for 30-06 if you're not pushing the loads hard.

With my .270 I tend to bump back the shoulders 2 thou and I expect 7-10 loadings out of my relatively mild 'plinking' loads and 5 out of the fairly hot hunting ones. I also usually find that the necks split before I get this issue.

There are plenty of others on here with more experience of course, but that's my opinion!

Al.

Thanks for the reply. That makes sense. I haven't had any experience of case head separation before so wasn't entirely sure what I was looking at but I did think it would be more obvious on the inside. I think it is just the die, the Redding die I use (only Redding die I use) seems to mark the cases more than the lee dies I use.
Other thing I have checked this morning is old brass I had from my other 3 06's and all the once fired brass measure the same to within .002", I would imagine that if the tikka had too much head space the brass would measure longer. Does this make sense???
 

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Makes sense to me.

As I say, I'm no expert, but I have seen similar on my own brass when doing the minimal sizing thing, so thought I'd share my thoughts.

Weirdly, I see more of this with my cheapy plinking Lee dies than my posh Forster ones, but I suppose it just depends on the tolerances and length of the die body and the brass quality? Remington was especially bad for me versus Norma. Doesn't seem to impact function though, so I can't say I dwelled on it overmuch.

The length of the fired case, ideally measured to the shoulder, not the rim, should give you a good idea of chamber dimensions, plus minus a thou or so for brass spring back, sure.

Al.
 

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It looks to me like the sizer die is just oversizing the case above the web. Or conversely, not sizing the case as far down the case as some.

Was the sectioned case fired (not sized), or sized?
What is the brand of the sizer?
Does it happen to the Lapua cases or just the WW?
 

James Adamson

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Makes sense to me.

As I say, I'm no expert, but I have seen similar on my own brass when doing the minimal sizing thing, so thought I'd share my thoughts.

Weirdly, I see more of this with my cheapy plinking Lee dies than my posh Forster ones, but I suppose it just depends on the tolerances and length of the die body and the brass quality? Remington was especially bad for me versus Norma. Doesn't seem to impact function though, so I can't say I dwelled on it overmuch.

The length of the fired case, ideally measured to the shoulder, not the rim, should give you a good idea of chamber dimensions, plus minus a thou or so for brass spring back, sure.

Al.

The brass was all measured to the shoulder.
 

James Adamson

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It looks to me like the sizer die is just oversizing the case above the web. Or conversely, not sizing the case as far down the case as some.

Was the sectioned case fired (not sized), or sized?
What is the brand of the sizer?
Does it happen to the Lapua cases or just the WW?

The sectioned case had been fired 3 times then cut before sizing for the 3rd time.
The die is a redding die and bother the lapua brass and the winchester brass looks the same.
 

James Adamson

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I am going to try some new brass and a lee die to see if the same marks appear after a few loading. I will let you know the results.
 

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In my experience there are two areas of the chamber/round interface that can cause what you are seeing. Headspace appears to have been eliminated. The second is the diameter of the chamber. If the chamber is toward the max dimension in diameter, which it not necessarily bad on a dangerous game rifle to improve feeding of dirty case; the case expands radially until you hit the thicker brass of the case base. When resized the case is squeezed down and has the die marking you are showing. This is really evident when a small base die is used to resize military surplus brass that has been fired in the "loose" chambers of military semi auto rifles. In really over diameter chambers this can be an issue and there will be a bulge on the fired case and that area may become over worked and prone to failure
 

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Thankyou all for your replies so far.
@fourfive8 very good information.

I am shooting a tikka T3x in 30-06. I have 3 different loads worked up for this rifle, a cheap practice/plinking load using winchester brass and 2 "good" loads using lapua brass. I measured and remeasured the brass again last night and all came out .003 to .004 of shoulder bump. After looking at it again this morning I am not sure if it is case head separation or just a mark left by the die. Here is a couple of photos, let me know what you think.

View attachment 343897 View attachment 343899
I had a 7mm Mauser that did that even with factory shells. I finally come to the conclusion it was a sloppy chamber. It never seemed to hurt much.
 

Ray B

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I agree with the Engineer. I think what you have is a chamber that allows the cartridge to expand. the web portion doesn't expand to the extent that the portion ahead of it does. So the die pushes the thin portion back while not needing to push the web. Since the die is resizing the thin portion it leaves wear marks. It isn't a problem other than the expansion/resizing will unnecessarily wear the brass.
 

bruce moulds

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JME, If you resize and reload the case multiple times, eventually the brass has stretched enough near the web that it will separate. That said, I have never had this issue with only 3 reloads. Usually in the 6+ reloads.
As mentioned headspace, but my experience is the very first round out of a 1991 Argentine Mauser notorious for headspace issues.
Too high pressure loads. Trying to get the last fps out of a load.

Best of luck figuring this out! Not a good situation!
those Argentinean mausers were noted for excess headspace.
the reason being they were soft, and set back, even with the original lower pressure loads.
if more people knew of this there would be few custom rifles built on them.
bruce.
 

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