Case Head Separation

K

kilotanker22

New member
Look what I found on eight of fifty cases after sizing tonight. All fifty have been fired the same number of times so I tossed the whole batch. These were Hornady 300 PRC cases with 8 firings. My chamber is snug, head space is .002" longer than the factory ammo cases were before being fired the first time. Shoulder is pushed back .002" to their original size. A few loads were a little hot, but no charge weights were above published maximum loads. I am getting velocities consistent with the published data. Primer pockets are still as tight as they were after the first firing. I cannot wait to be able to buy some Lapua, Peterson, or ADG cases for this rifle. Buying factory ammo to get brass is way more expensive than premium brass haha.
 

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Did you mean that the shoulder was pushed back 2-thou from fired to original factory dimension?
Or that shoulder was pushed back 2-thou from bolt just barely closing?

(Big difference between the two)
 
I'd double check that you are in fact sizing how you think you are. Measure, and double and triple check. It never hurts to physically re-verify everything.

What are the tools and procedures you're using as well?

Not that I think you're doing anything wrong or anything, it very well could be bad brass. I'd just hate for you to chalk it up to that and have it be something simple that's overlooked and have you ruin good brass

Sent from my SM-G996U using Tapatalk
 
mehavey said:
Did you mean that the shoulder was pushed back 2-thou from fired to original factory dimension?
Or that shoulder was pushed back 2-thou from bolt just barely closing?

(Big difference between the two)
Click to expand...
When firing factory ammunition, the shoulder is only pushed forward .002". When I size them, I push the shoulder back .002". This puts the base to shoulder dimension back to its original length from factory, minimum sized cases. When fired again, they grow .002" and the cycle repeats. Believe me, I have measured every single case after every loading, trimming and sizing, every time. I was just surprised to see case head Separation with the minimum movement of the brass.
 
nhyrum said:
I'd double check that you are in fact sizing how you think you are. Measure, and double and triple check. It never hurts to physically re-verify everything.

What are the tools and procedures you're using as well?

Not that I think you're doing anything wrong or anything, it very well could be bad brass. I'd just hate for you to chalk it up to that and have it be something simple that's overlooked and have you ruin good brass

Sent from my SM-G996U using Tapatalk
Click to expand...
I double check the dies, and all other tools every time. Head space is measured both before and after sizing each case. Sometimes I choose to measure every case, other times I only measure about a third of them. In every round of case prep I inspect every case for split necks and head separation.

Dies are Hornady Custom grade FL die and seater. Press is the newest version Rock Chucker Supreme, proper shell holder. I use a Little Crow Gun works WFT 2 trimmer, Lyman ELD chamfer tool and RCBS debur tool, and RCBS brushes for primer pockets all run on my RCBS Case Mate
After firing every case is cleaned, annealed, lubed, sized, cleaned again, then chamfered, debur and clean primer pockets. At this point every single case is inspected for issues and a third of them are selected for scrutiny by measuring all relevant dimensions. Occasionally, I will choose to measure every single case for all relevant dimensions.

Primers are CCI #34, hand seated to .006" below flush with the head. This is the anvil bottoming out in the pocket plus about .003". Notes are carefully kept on each load tested, date, temperature etc. No bullets were anywhere near jam and no loads were even within a grain of published max loads. Every single charge is dropped then trickled to within .02 grains of my nominal charge weight.

I have had no signs of excessive pressure, no loss in accuracy, no signs from the rifle that something was amiss.

Let's talk about the rifle. Browning X-Bolt Max Long Range. The only modifications to the rifle are lightening the trigger weight, and the bolt lugs have been lapped because one lug was not making full contact inside the receiver. Barrel was lapped prior to this batch of brass being fire formed, after which my headspace measurement was taken again from the fire formed brass where I found no change in the base to shoulder length on fired brass. Fired brass are decapped prior to measuring the base to shoulder to prevent the primer from affecting the measurement. The measurement is also taken after the brass has been cleaned. The rifle bore and chamber are cleaned to the bare metal after each range visit to aide in the bore always being in the same condition when testing new loads. I usually fore one or two shots to foul the bore before testing.
 
I believe most of us know that case head separations are not caused by excess pressure but by excess headspace, upon firing the case walls grip the chamber walls and the case head moves back into contact with the bolt face. The case walls thin each time the case is over-sized and fired, eventually separating after enough cycles.

In spite of all the measuring, thats’s the only way I can see this happening given the explanation. One thing is for certain, it isn’t magic.





.
 
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What tools are your using to measure headspace and shoulder bump though?

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So, you wore out your brass, what's your point??

You said you got 8 firings out of them, I think that's pretty good for a high intensity rifle round where you are working the brass hard.

What were you expecting and why?

Brass doesn't last forever, and you got 8 firings, what's your issue?
 
OP said:
When firing factory ammunition, the shoulder is only pushed forward .002".
When I size them, I push the shoulder back .002".
Click to expand...
Do me a favor:

Unscrew the sizing die a turn+1/2 (way clear of the shoulder)
Start sizing a fire case, chambering it, and then size again +1/8th turn (or less).
Repeat.

Sooner or later the case will have squeezed to a longer length to where the bolt starts having trouble closing.
What's the comparator measure as case headspace dimension at that point?



postscript: Use that point as reference to push the shoulder back 2-thou, not the as-fired size.

.
 
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So, you wore out your brass, what's your point??
Click to expand...

Two thing are of interest to me .

1) He only got 8 firings with a .002 bump . I get more then that with a .003 bump with my AR's . So somethings up there

2) Goes to 1 If he's truly only bumping .002 He should not be getting head separation period .

If only bumping .002 from fire formed he might get split necks or blown out primer pockets after 5, 6 , 7 or 8 loadings but not case head separation . The only time I've had case head separation was when I first started and didn't understand the idea of shoulder bump and sized cases per die instructions . This resulted in me setting the shoulder back .010+ and only got 3 reloads before case head separation .

KT22 how much do you need to trim after each sizing ?

EDIT

After giving it some more thought and rereading the OP I think I figured it out .
OP said:
My chamber is snug, head space is .002" longer than the factory ammo cases were before being fired the first time.
Click to expand...

If you are basing all your comparator measurements off of once fired in your rifle factory ammo .That might be the problem , some times neck sizing only and firing a second time gives you a more true fire formed case . Lots of times factory ammo is not hot enough to blow out the case enough to get a true fire formed measurement . That's likely the issue but second - did you deprime before making your measurements of this once fired factory ammo . Spent primers still it the case can give false measurements do to cratering or them being pushed out a tad . Either one of those thing can throw you off , combine the to and you can be WAY off of what you think is a .002 bump .
 
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Metal god said:
Two thing are of interest to me .

1) He only got 8 firings with a .002 bump . I get more then that with a .003 bump with my AR's . So somethings up there

2) Goes to 1 If he's truly only bumping .002 He should not be getting head separation period .

If only bumping .002 from fire formed he might get split necks or blown out primer pockets after 5, 6 , 7 or 8 loadings but not case head separation . The only time I've had case head separation was when I first started and didn't understand the idea of shoulder bump and sized cases per die instructions . This resulted in me setting the shoulder back .010+ and only got 3 reloads before case head separation .

KT22 how much do you need to trim after each sizing ?

EDIT

After giving it some more thought and rereading the OP I think I figured it out .

If you are basing all your comparator measurements off of once fired in your rifle factory ammo .That might be the problem , some times neck sizing only and firing a second time gives you a more true fire formed case . Lots of times factory ammo is not hot enough to blow out the case enough to get a true fire formed measurement . That's likely the issue but second - did you deprime before making your measurements of this once fired factory ammo . Spent primers still it the case can give false measurements do to cratering or them being pushed out a tad . Either one of those thing can throw you off , combine the to and you can be WAY off of what you think is a .002 bump .
Click to expand...
As stated earlier I decap prior to measuring. And every piece of brass is annealed after every firing, the measurement has not changed.

I am using the Hornady anvil and head space comparator. It's not difficult to grasp the concepts we all discuss and use regularly.

In the first batch of brass (before this batch) I did have some hotter loads where I had some heavy bolt lift and a stuck case. The stuck case base to shoulder measured 2.194". Hornady factory ammunition measures 2.189". Fire formed cases both factory ammo and my loads that did not stick measured and still measure 2.191". Cases get sized so the base to shoulder measurement is returned to 2.189". Cases that experienced heavy bolt lift measured a very small fraction over 2.192".

I track my shoulder bump and case to shoulder dimensions closely and record these measurements every loading. That way If I run into an issue I can review what I have done to the brass this far.

Each case only needs a few thousandths trimmed off each time. I trim them back to 2.565" after every sizing. Usually the cases after being fired and sized increase in length by between .003-.005".

This is the same process I use for many other calibers and it works well. I am sure of my measurements.

To answer the fella that asked my point. I don't really have a point other than surprise at the notion of case head separation, given how little the brass is being worked. Considering that this is not what I would call premium brass, and I got eight firings on them I am not really disappointed. The purpose of this thread was only to share what I found. And to share the message that case inspection is still important, even if your methods are solid.
 
Not that bad, considering what the cartridge is.
Hold it up to a comparable cartridge, like .338 WM, and you're at a similar failure point.
Brand doesn't matter much for .338 WM. Six to eight, and they're toast.

But if you want something to think about, try neck sizing, not shoulder-bumping.
If the case will chamber, there is no point in moving the shoulder. (Which, of course, has an impact on more than just the shoulder.)
 
Understood, I was just concerned because you mentioned factory ammo and how it compared to the fired cases and you returned the cases to factory spec . When I’m sizing/bumping the measurements of the round before it’s fired is of no concern to me as it relates to finding out how much I will be bumping that shoulder after firing . That’s all I was getting at , I must have misread the meaning of the OP in that regard .

If you’ve got the sizing all figured out that leaves what was described in the last post . That cartridge is hard on brass which then brings me back to 44’s point . What’s the question then ?
 
Despite this being a magnum pressure and performance round, it is interesting to note he's not getting unusual case growth, so I don't think the dies are narrowing the cases excessively, and he isn't setting them back very far. So while 8 reloadings of a hot load is a fairly normal number, it is still interesting to contemplate what the dynamic differences are from, say, a 270 Winchester case (also a 65,000 psi round) and this one.

It would be interesting to section one of the tossed cases to get a look at how long the pressure ring area is. I am guessing it would reveal, as compared to any of his other guns, that the pressure ring stretched area is short and the transition to normal thickness areas is fairly abrupt. This means getting longer case life would entail spreading that stretch over a greater length. This is actually possible to do by polishing the bottom half-inch to an inch of the chamber to reduce the grip that pressure gives it against the chamber wall. That can create enough slip to spread the stretching spread it out a bit. You could probably use the Flex-Hone 400 and 800 grit tools intended for all the 338 magums to get the surface toolmarks off that lower length of the chamber. A bore mop and Iosso Bore Paste would rough polish and Flitz or Dico's blue SS polish would final polish the lower part all using a drill like the hones do. It's a bit of bother, but you can see the effect simulated in Varmint Al's animations of a 243 firing. Those and Slamfire's experiments with firing lubricated 30-06 show the change in bolt thrust is not as great as some expected, even with the whole chamber polished. So a quarter to a half chamber polished wouldn't cause exceptional gun stress. Anyway, something to consider.
 
You use the factory round as reference, which you believe has 2 thou head clearance in your rifle. Measuring that 2 thou clearance is the key.

I haven't loaded 300 PRC myself. 8 loads isn't too bad if it is a high pressure cartridge. Pressure surely is a major factor in brass life, head separation included. My brass regularly last over 20 firings, partly because I don't do hot loads often. Usually they have split necks before head separation. My way to minimize head clearance is same as meharvey's; partial length resizing. Keep doing it till resistance in closing action. Then thread the die down 1 thou at a time till resistance disappears. I don't own, almost, any fancy measuring doodad.

You are in an internet forum. I'd take in any "speech" with a grain of salt. I remember an expert here claimed that the firing pin impact alone would move the shoulder back by up to 10 thou. That basically puts all these discussions moot. You are going to have minimum 10 thou head clearance regardless how careful you resize. I don't believe it at all.

-TL

PS. Slamfire, another member here, suggested greasing brass to avoid head separation. It is unconventional, but I believe it works. I do that sometimes in case of excessive head clearance. At least I don't clean resizing wax off my brass.

Sent from my SM-N960U using Tapatalk
 
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Consider this, when a case is fired the pressure pushes out in all directions. The brass will move in all directions (forward, back and expand in diameter) until it is stopped by the steel of the gun. Shoulders move forward until stopped. Case heads move back until stopped. Case walls move out, until stopped.

This stretches the brass. So, some point of the case gets thinner. With bottle necked cases that point is usually just above the solid case head. Resizing, (full length or partial) does not put the brass back where it came from inside the case. It just changes the outside dimensions.
Over time, (loading/firing cycles) the thin spot gets thinner, eventually reaching the point of failure. Annealing doesn't change this. Careful measurement with a comparator doesn't change this. Ensuring uniform shoulder set back in sizing doesn't change this. Nothing changes this it is the nature of the beast, and is always going to happen.

What can, possibly. be changed is how soon (loading cycles) this happens, and that depends on a host of factors that are individual to your rifle, your brass, your dies and your loading process.

I've had a complete case head separation on the "second" firing of a .303 British case. (at which point I decided "once fired" .303 British brass wasn't worth the cost savings). I've had .308n Winchester cases I've loaded over a dozen times, FL sized with my 1970s Lyman sizer die hard against the shellholder without any signs of case head separation. Does this mean you will, using the same cartridges? NO! You might, you might not, that is something the individual factors of what you are using will add up and determine. You might get more case life from "high quality" brass, you might not.

You're shooting top end loads in a high intensity cartridge. No matter what your sizing methods are, you're still working the brass hard every time it fires. Eventually, it will wear out. Exactly when is something unique to your situation and can only be determined by use.

I've known people who use their brass until it actually cracks. And I've known people who toss out their brass after 5 loadings, just as a matter of principle. What you get is what you got, and that can be slightly different, or vastly different from everyone else, due to many, many factors.
 
If I understand you correctly, you're taking you initial measurements off a new unfired case. Try the following. It might make a difference.

This is how I set up my sizing die for bottleneck cartridges.

1. Take a once fired factory round and blacken the neck and shoulders with a Magic Marker or Sharpee pen. Some people like to smoke the neck and shoulder, but I find the Magic Marker/Sharpee pen a bit better.

2. Carefully lubricate the case.

3. Loosen the lock ring on the sizing die and back off about two turns from when the die is set to touch the shell holder.

4. Size the case. Note where the marks are on the case and turn the die down about a half a turn and size again. Turn down some more, and resize again. What you are looking for is the marks on the blackening just touching the shoulder.

5. Clean the lube from the case and try it in the rifle. It may chamber just a bit on the snug side. If so, turn the die down ever so slightly, lube and size again. Wipe off the lube and try in the rifle. If it slides in as easily as a factory round, you should be good to go. If not, usually one more very slight adjustment should fix the problem.

6. Tighten the locking ring for the die and you're done. You have just set your sizing die up for a custom fit to your specific rifle, rather than a generic one size fits all guns.

Paul B.
 
This thread is interesting . The OP is certain he is sizing his cases Correctly . Why are so many trying to fix things that ain’t broke ? FWIW there might be a bit of sarcasm here haha
 
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