What causes head separation in bottleneck rifle cases

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Metal god

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Ok , I’d like to know what specifically causes it and how it happens . We all know if you size your case to short ( head to datum point ) . Repeated firings with a case sized to short for the chamber you will get case head separation . Why ?


I believe I understand why but this question came from another thread where Jeephammer (another member here) and I disagree on how casehead separation happens .

Short version of the debate is

I believe the firing pin strikes the primer shoving the case fully forward . The resulting ignition causes the case to expand sticking it to the chamber walls BEFORE the case has a chance to move back to the bolt face . This results in a small gap between the bolt face and head of the case . Now if overall pressures do not get high enogh to stretch the case head back to the bolt face . The primer will be pushed out that same amount as the gap and the bolt face stops the prime . When you eject that case the primer will be protruding from the primer pocket . When you see primers sticking out , it’s a classic sign of a low pressure load and increacing powder charge is likely needed ??????

How ever when the firing pin hits the primer shoving the case forward and the case expands sticking to the chamber walls leaving that gap between the head and bolt face . If the pressures are high enough the case head it self will stretch at the web allowing the head to move back against the bolt face . It’s this stretching at the web that thins the case wall resulting in casehead separation if you continue firing cases that have to much gap between the bolt face and head of the case because each time the case is fired the web stretches and gets thinner and thinner until it splits ?????

I’ll let JH explain his theory in detail but in short he believes after the firing pin strikes the primer shoving the case fully forward the resulting pressures push the case back against the bolt face before the case expands enough to lock it against the chamber walls . Again he can explain better but I believe his theory is the the force of the shoulder being blown out and forward stretches the web of the case .

So in short I believe case head separation is caused by part of the case being forced rearward and JH believes it’s caused by part of the case being forced forward .

Forgive me JH if I did not correctly explain your point of view .

So how does case head separation actually happen

P.S. Forgive me if there are a bunch of type-o I’m on my phone and it does not make for the best typing .
 
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The case doesn’t fit the chamber, either case sized right and chamber too large (like will chamber a “no go” gauge) or chamber right and case sized too much.

Either way they blow out each time and you have to keep trimming the excess after sizing, at least until they spilt into.
 
Ok , I’d like to know what specifically causes it and how it happens
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I do not believe you will ever get there, I believe you are too selective. Reloaders claim they can move the shoulder back, they claim they can bump the shoulder back; I claim it is impossible to move the shoulder back with a die that has case body support. And then there is that thing about ever thing having head space. Not me, my cases do not have head space.

And then there is that story about the firing pin driving the case, bullet and powder forward until the shoulder of the case collides with the shoulder of the chamber. I do not know of many reloaders that want to spend the time necessary to understand what happens when the trigger is pulled.

F. Guffey
 
I don't subscribe to case 'Sticking' to chamber walls enough to overcome 50k-60k pressure the bullet will produce when it hits the throat/rifling.

The bullet is a 'Plug' in the barrel, the case is supported at the neck and sides and the only direction the case can move is backwards.
As soon as pressure increases enough to move the bullet in the case, overcoming neck hold (and/or crimp), the case has no choice but to move rearwards.
This is a basic law of physics, the bullet is moving forward, the opposite & equal reaction is the case moving rearward.

Even the most oversized case stuffed into a sizing die can be pulled out with a couple hundred pounds of force produced by any press.
When compared to 50k-60k of chamber pressure, the press is nothing and still pulls the most wedged in case out of the die.

Keep in mind here that cartridge brass starts to go 'Plastic' (starts to 'Flow') between 65k and goes fully plastic at around 80k. 50 ton (100k) press will stamp out cases without hesitation rings all day long.


As for under loaded, 'Light' charges backing a primer out...
The ONLY times I've seen a primer backed out was in blowback or gas operated firearms with short barrels (short barreled AR clones).
Others might have different experiences, but I've never seen a primer back out in bolt or pump rifles with locked bolts.

Also, keep in mind that seriously UNDER case volume, there are several credible sources saying that primer sets more powder on fire than it's supposed to, spiking chamber pressure before the bullet mass can get moving.
There is a reason it's OK to compress a lot of powders, but filler is recommended when the case volume isn't filled most of the way up.

This is particularly true with black powder, which can't safely be compressed.
Berdan primers were actually developed to keep primers from 'Flashing' the surface of partly filled cases. But that's neither here or there with modern powders, just a historical footnote.

Now, keep in mind, the hottest part of powder burn happens in the back of the powder chamber, right in front of the case head. The temprature is intense, and since the bullet (mass) hasn't started to move yet.
This makes pressure spike, and when you see a graph of pressure, you see this.

Keep in mind the powder often 'Clumps' under moderate pressure, anyone that's pulled a compressed load down knows this, clumps are always fun to get out of the case.
So you have the mass of the bullet getting forced into the throat/rifling, AND you have a clump of unburned powder stacked up against the SHOULDER and the bullet.

Again, all that pressure is applied to the head of the case, and with the incredibly high temprature in the back of the case, it will carbonize the brass, but ONLY in the back of the case.
We've all seen the cut-aways of case that have been fired several times, scale carbon on the surface will come off, while carbonized brass is impossible to clean.
(I've steel pin cleaned the crap out of carbonized brass, it wouldn't come off, so I put it under magnification)

So we KNOW there is enough pressure & temperature contained in the back of the powder chamber to very nearly make brass 'Plastic'...
We know case heads usually separate just above the web at the back of the powder chamber...
We also know the case is blown backwards with the entire recoil force, simply because recoil will be in the opposite direction the bullet travels...
We also know that any press producing a couple hundred pounds of pressure will crush a primer into flattened junk...
Under normal circumstances pressure, the primer gets flattened out against the bolt face, it's quite easily deformed..
We've all crushed primers at one time or another without even trying.

*IF* the case 'Sticks' and pushes the primer out against the bolt face, wouldn't it be mangled?

How is it gently backed out intact without being crushed stupid?

This sounds a lot like the case slams into the bolt & the bolt moves backwards slightly, the primer being recessed, keeps moving via momentum until it hits the bolt face and stops, leaving it mostly intact but backed out slightly.
(This explains why *I've* never seen a backed out primer in a solidly locked bolt action, but I also don't shoot anything with excessive horizontal slop in the chamber)

A half filled case would pressure spike when horizontal position with too much powder igniting all at once,
BUT,
You have to consider the flash hole works as a pressure regulator.
Pressure being a function of volume, and through a restrictive hole like the flash hole, the primer wouldn't see but a fraction of the pressure in the case.
This is why you have to be GROSSLY overpressure to blow out a primer pocket...
The volume needed to build pressure in the primer pocket can't easily enter through the flash hole.

I don't know for sure, but I don't subscribe to the case 'Sticking' in a way over length chamber, or a WAY short case that would allow a primer to nearly back out of the case.
This is just too many things that would have to be WRONG, and all of them line up precisely,
And it flies in the face of physics to boot.

No one is more aware of how abrasive brass is, and I'm perfectly aware it can jam up with a death grip WHEN COLD in dies. Trust me on this!
I'm also aware the workability of brass is increased exponentally as it heats up. I can't see hot brass NOT slipping rearwards under the tremendous pressure rearward pressure on the case.
The way I see it, the physics don't support the case sticking during the entire firing cycle, and when the brass does release the bolt face would simply shove the primer back into the case.

It's up to you to believe what you want to...
 
Even the most oversized case stuffed into a sizing die can be pulled out with a couple hundred pounds of force produced by any press.
When compared to 50k-60k of chamber pressure, the press is nothing and still pulls the most wedged in case out of the die.
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OK , when you are pulling a case from the die there is no extra friction being forced against the die walls . How ever in a chamber the 60K psi is being applied equally in all directions . So how many sq in does a 308 case have ?? I have to say at least 3sq inches , which to me would mean that 60k psi is actually 180k psi of friction between the case and chamber wall stopping it from moving backwards ????

As for under loaded, 'Light' charges backing a primer out...
The ONLY times I've seen a primer backed out was in blowback or gas operated firearms with short barrels (short barreled AR clones).
Others might have different experiences, but I've never seen a primer back out in bolt or pump rifles with locked bolts.
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This has not only happened to me on a few occasions . I've seen it at the range from other reloaders asking me what it meant as well as reading about other reloaders having this problem in multiple threads on multiple forum boards . I'm 100% convinced it not only can happen , I know it happens . We can debate the cause but it happens .

Also, keep in mind that seriously UNDER case volume, there are several credible sources saying that primer sets more powder on fire than it's supposed to, spiking chamber pressure before the bullet mass can get moving.
There is a reason it's OK to compress a lot of powders, but filler is recommended when the case volume isn't filled most of the way up.
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I'd have to go dig out my notes from awhile ago but if I recall . My loads were not overly low for 308 and in fact were Hornady's start load using 178gr A-max and either IMR 4895 or 4064 . It's possible it was using 190gr smk and N-540 . I also have a clear memory of the guy next to me at the range shooting a 30-06 using IMR 4895 having his primers backed out after ejection , don't recall the charge or bullet .

*IF* the case 'Sticks' and pushes the primer out against the bolt face, wouldn't it be mangled?
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Correct , and that's a direct result as to why flattened primers are an indication of over pressure . On a severely over pressure round the primer pops out a tad and actually starts to mushroom . Then as the case head stretches back and is forced back against the bolt face it reseats the primer but the mushroomed part gets kinda pinched and smashed flat by the case head . Someone here has a link to or an illustration of this very thing .
 
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Oh, for heaven's sake.

Envision a bottleneck cartridge from inside.

There is a case head that will - when ignited - push against the bolt. There is also a case neck that - as pressure rises - will be pushed against the "neck" in the chamber.

The case neck is at an angle which means that when the powder ignites, the case head and the case neck are forced in opposite directions towards the case head (and the bolt) and towards the neck of the chamber (which stretches the case forward against the shoulder in the chamber).

The brass between the neck and the case head must be stretched to conform to the chamber of the rifle that fired it.
 
Simple pressure equations will tell the story.
Pressure acts in ALL directions at once. The law of for every action is an equal and OPPOSITE reaction does not apply, except in regard to recoil.
Chambers are roughened after cutting with either scotch brite pads or 600 grit emery cloth.....the reason, so the cases STICK better. You won’t find too many people advocating highly polished chambers.

When a cartridge is fired, the firing pin DOES force the case forward, then as start pressure balloons the forward thinner section of the case, the shoulder and neck seal the chamber, then when max pressure is reached a millisecond later the entire case is thrust against the walls of the chamber so tightly that the rear of the case above the web stretches to meet the bolt face. If you look at fired cases, the expansion line is ALWAYS larger than the solid head section and if you watch the expansion line as you increase powder charges you will see that the expansion, and therefore stretching, takes place CLOSER to the web.
The only way to make a case move rearward in the chamber is to grease or oil it.
NATO do this when pressure testing their ammo, it gives them a true reading of not only cartridge pressure, which they measure at the case mouth, but also the true bolt thrust, even though it is far less when the cartridge is NOT greased/oiled.

Cases do not move rearward when fired, nor do they stretch to fill the shoulder from the web, if they did every belted magnum would fall apart on the first or second firing. Moving the shoulder forward .026” on average DOES NOT stretch the web in belted mags, cause the belt controls headspace on the first firing.

The only cause of incipient case head separation is excessive headspace, whether a gun problem or induced by oversizing the brass. Yes, this applies to belted magnums, as the case still stretches upon each firing, just like any other case type.

Cheers.
 
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The brass between the neck and the case head must be stretched to conform to the chamber of the rifle that fired it.
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Why is it that this area only is where the case stretches , gets thinner and cracks ?? It's not thinner then the rest of the case forward of the web so why does it only get thinner there ? How come on some rifles it does not thin out a inch higher then that ? It would seem reasonable to conclude if the case head is pressed hard against the bolt face . The case could stretch and thin anywhere ahead of the head/web and yet is always stretches and thins in the same place on every head separation ???

G9JFNo.jpg


I'll add these were mild loads that were reloaded only three times . The issue was they were sized to have a casehead clearance of .008 to .011 on each firing , oops :o
 
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Primers sticking out after firing

I went ahead and looked through my older data . I found three different loads that had primer protruding after firing . There may be more but I stopped looking after I found three , figured that was enough .:)

1) 9-14-13 , All 5 primers backed out
178gr A-max
40.0gr Power Pro 2000mr Hornady's start load is 39.9gr but Alliant has 43gr as start load
Fed cases
Win LR primer
COAL 2.790

2) 9-18-13 , 3 of 5 primers backed out
178gr A-max
36.5gr IMR-4895 , Hornady start load is 33.8gr
Fed cases
Win LR primers
COAL 2.780
Ammo temp 79* barrel temp 80*

3) 10-2-13 , 2 of 5 primers backed out
175gr smk
38gr IMR-4895 , Sierra start load is 36.7gr
Remington cases sized with 336 bushing
Win LR primers
COAL 2.785 .

FWIW the powder was from a 8# jug I loaded many very consistent loads with over the couple years it was around . I'm still using the same lot Win primers that came from a 5000ct case with no issues . The rifle now has well over 3500rds down the pipe which I've posted many a small groups I've shot . The point to that . The equipment and components did and continue to work great .

You may see a pattern of the COAL being short . The rifle I was using was new and had a pretty short throat and I want to keep the bullet way off the lands . Regardless there is my proof at least in my eyes . ;)

Oh the other pattern you may see is I clearly had nothing going on in my life in September of 2013 seeing how I was pretty much shooting every week lol , I was likely just stoked to work up loads for the new rifle .
 
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It varies with case/chamber dimensions and surface finish of the chamber, but as I recall, the math shows 2 to 3 times more force holding the case body against the chamber walls than the force against the case head (due to differences in surface area). So it happens as Hornady describes.
 
Metal God,

You are both right in different instances. It just depends on how fast pressure rises during firing. Lower peak pressure cartridges (slower rate of pressure rise) do back up, as JeepHammer describes, but higher pressure cartridges (faster rate of pressure rise) do what you describe because pressure rises and sticks the brass to the chamber wall faster than the inertia of the bullet and case masses allows them to start moving apart. In other words, in the high-pressure case, the brass is stuck to the chamber wall while the cartridge still looks like an assembled, if swollen, cartridge.

If the higher pressure cartridges backed up in the chamber like the lower pressure ones do, the shoulder, being thinner brass than the pressure ring area near the head, would expand first, fireforming the shoulder forward so that all the stretching would occur there and not near the head. A shoulder blowing forward pulls brass from the neck as it expands and resizing (moving the shoulder back) just returns it to where it was originally, so no net case growth occurs. For this reason, whether or not you need to trim your cases tells you which side of the pressure rise rate line your load is falling on. The backup side or the stick and stretch side.

Hartmut Broemel, who authors QuickLOAD and who is one of Europe's leading ballisticians (he authors the CIP's internal software), explains this in the QuickLOAD manual. Roughly speaking (and this varies with the powder's pressure curve shape and the case shape) he puts the dividing line at about 30,000 psi peak pressure. Cartridges fired at peak pressures above that number are more likely to stick their case walls to the chamber before the inertia of the bullet and case can be overcome. Cartridges fired below that pressure tend to back the whole case up and no net stretching occurs, so the cases don't need trimming.

In both the high and low-pressure instances, after ignition, the primer backs out under a combination of its own pressure and the rising pressure inside the case, which extrudes it around the firing pin before the pin backs up or is pushed back (floating firing pin). The head moving to the rear, whether by stretching (>≈30,000 psi load) or by case backup (<≈30,000 psi) then reseats the primer. If the primer is sticking out, the pressure never got high enough for a long enough time to stretch the head or to back up the case.

There are other factors. A bottleneck case has more grabbing surface area than a straight case for the same caliber bullet does. As a result, relatively straight cases, like pistol cartridges often fire a bit above 30,000 psi without stretching noticeably, while a downloaded bottleneck case may stretch at that same pressure. So the 30,000 psi number is not absolute. It's more the vicinity of a range of pressures over which stretching may begin to happen.

The reason case sticking happens at the higher pressure boils down to the old problem of analyzing what is happening as a dynamic rather than a static event. If you calculate what is happening at fixed pressures or very gradually increasing pressure, you would find that a 30 caliber bullet with about 60 lbs of bullet pull (new cartridge military ammo spec) would start to move out of the case when the pressure in the case reached about 800 psi (60 lbs divided by the cross-sectional area of the bullet of 0.0745 in²). But in a dynamic analysis, where pressure rise is happening in the time frame of tens of microseconds, moving a bullet in that short time isn't just a matter of reaching 800 psi. What static analysis fails to consider is the bullet's mass and the case's mass have inertia that has to be overcome to accelerate them apart from one another. The shorter the time frame, the higher the pressure needed to achieve a particular length of movement. As a result, in a high-pressure load you see pressure in the case rising enough to expand the brass against the chamber before the bullet or the case move significantly.

The proof of that last point, in addition to the need for trimming, is curled case mouth lips and gas bypass. When the brass expansion pressure is reached, the back of the neck near the bullet starts to expand before the bullet starts to slip inside it. This results in the case neck peeling away from the bullet rolling forward from behind until the leading edge of that neck expansion reaches the case mouth. At that point, gas starts to leak out around the bullet and into the bore. That leak causes a pressure drop along the sides of the neck, so the neck never completes expanding near the mouth; it only expands completely along its sides below the mouth (see case mouths in the photo below). This is why you often are unable to drop a bullet into a fired case without interference by the mouth.

Super slow motion photos of bullets leaving the muzzle of a gun show the gas that has bypassed the bullet coming out first. Some claim that is just air compressed in the bore, but a high power rifle bullet is typically traveling near Mach 2 and has a shorter barrel time than the time it takes sound to travel the barrel length, so the bullet catches up to merely compressed air moving forward at a maximum of Mach 1. Thus, if bypass pressure were not driving the gas out ahead of the bullet, that air, like a shockwave bow, would only be a small fraction of an inch out in front of the bullet.

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I'm no expert, nor am I an engineer versed in the esoteric minutae of pressure calculations. But there are a few things I've seen over decades of observation.

I believe the firing pin strikes the primer shoving the case fully forward
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I believe this also, BUT, only if the case has somewhere to go, and nothing else stops it. Meaning, there has to be space for the case to move forward into, and nothing like an extractor, case belt or RIM preventing that.

The design and tolerances of the action, case, and chamber together determine things like that. Now, I realize we are talking about very small distances, and I can see where the firing pin of a push feed action could drive the case forward until something stops it (assuming there is room for it to move forward) but from where I sit, it seems that that amount of movement would be different in a controlled feed action such as a Mauser 98, where the claw of the extractor is already in contact (or very, very close) to the leading face of the case rim, preventing the case from being driven forward.

And, what about rimmed cases??? (and belts are essentially rims). When the rim of the case is tight against the headspace ledge in the chamber, how can it be driven forward any further than that ???

Take for example a .303 British, or a .30-30, and lets assume we have good headspace (for the RIM) nice and tight and the case rim cannot and does not go anywhere when fired. But the rest of the case does "move" (expand) into the space available. Sometimes, especially with .303s, that's a LOT of space, relatively speaking.

We still get case head separation over repeated firing, reloading, firing cycles. In the .303, with its "generous" chamber dimensions forward of the case rim (in SMLEs) there's a lot of case stretch, sometimes enough that cases fail as early as on the SECOND firing cycle.

Another point is that primers have enough force all by themselves to back out of cases, if there is room for that to happen. The easiest place to see that is in a revolver (where there is intentionally LOTS of empty space behind the case head). Load up 6 primed cases, and try to pop them all off. If at least one of the primers doesn't back out (and maybe enough to drag on the recoil shield, its a VERY rare occurrence.

Do the same thing again, but put a paper wad in the case, and that doesn't happen. The paper wad creates enough backpressure that the case backs out the expected amount, reseating the primer.

Pressure inside the case pushes in all directions. The case moves (stretches) in every direction where there is room for it to move into. Whichever part of the case reaches that "hard stop" first, will stop, and the rest of the case will be forced where ever it can go, until it too reaches a hard stop.

Whichever end stops first doesn't matter too much I think. The result is the same, the case gets thinner in the middle.
 
And, what about rimmed cases??? (and belts are essentially rims). When the rim of the case is tight against the headspace ledge in the chamber, how can it be driven forward any further than that ???
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I have said many times reloaders have no clue as to the events that happens between pulling the trigger and the bullet getting past the rifling. And; I have said manufacturers of components do not sell components to reloaders that know what they are doing. I know, that is confusing to most but most reloaders are too selective about what information they will accept for silly reasons.

F. Guffey
 
"...pressure the bullet will produce..." Isn't the bullet that produces the pressure. It's the burning powder that creates the gases that do that. And it's the pressure that causes the case to expand, in every direction, to seal the chamber.
And the firing pin doesn't shove anything anywhere. It merely crushes the priming compound causing it to ignite. Thus starting the entire process.
If the headspace is excessive, the case will stretch too much and over several firings the case will eventually stretch so much the case head comes off. That's just one reason. Multiple firings will eventually do it too.
 
The issue doesn't change much with rimmed or belted cartridges. SAAMI cartridge and chamber drawings indicate the 30-30 can have between 0 and 0.017" of rim clearance between the bolt face and chamber face. For the 7mm Mag, the belt "clearance" is from 0 to 0.015" from bolt face to rim step in the chamber.
 
44 AMP said:
Meaning, there has to be space for the case to move forward into, and nothing like an extractor, case belt or RIM preventing that.

The design and tolerances of the action, case, and chamber together determine things like that. Now, I realize we are talking about very small distances, and I can see where the firing pin of a push feed action could drive the case forward until something stops it (assuming there is room for it to move forward) but from where I sit, it seems that that amount of movement would be different in a controlled feed action such as a Mauser 98, where the claw of the extractor is already in contact (or very, very close) to the leading face of the case rim, preventing the case from being driven forward.
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I believe this is a case with a great many rifles. I'm no expert on various ejector, extractor combinations/designs, but there's certainly no shortage of them that have a spring-loaded ejector that keeps the case pushed forward either until it stops against the shoulder (or rim) or the extractor.

I'm also quite certain that the same behavior can be a contributor to poor(er) accuracy, since in many instances the cartridge will be at a slight angle in the chamber.

The description given by UncleNick is the standard explanation I've seen from every expert source I've ever read and, in essence, I believe it's fact. There are certainly variables, as we've described, but explanations on any topic which attempt to explain every possible exception to the rule can get quite unwieldy, to say the least.
 
it seems that that amount of movement would be different in a controlled feed action such as a Mauser 98, where the claw of the extractor is already in contact (or very, very close) to the leading face of the case rim, preventing the case from being driven forward.
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What is the tolerance of a controlled feed action/extractor ? I can't imagine it's .010 or less which leads me to believe there is plenty of room for the case to move forward .

You bring up a good point about the 303 and I'd assume the 762x54r have case head separations . I'd again ask what is there headspace tolerances . By looking at the SAAMI drawings it sure seems like there can be a huge amount of case movement in the 303 British . I have to believe when loading for those cartridges and others that have rimmed bottleneck cases or belted cases . It's best to size them so they actually headspace off the shoulder by bumping the shoulders back only .002 rather then FL sizing which likely bumps the shoulders WAY more then that .

I believe this is a case with a great many rifles. I'm no expert on various ejector, extractor combinations/designs, but there's certainly no shortage of them that have a spring-loaded ejector that keeps the case pushed forward either until it stops against the shoulder (or rim) or the extractor.
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I'd have to assume the ejector of a push feed and or many other action types keeps the case as far forward as possible . The over all point is when the cartridge is fired is there a gap between the case head and bolt face . If so and that gap is more then .004 or .005 on each firing that web area is going to stretch and thin out ?????
 
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While I think 3" square surface area might be stretching it, I'd say it's a workable figure, so we'll go with that...

Now, as for the idea of 60 grit (or whatever) chamber walls to 'Grip' the case, not me, not ever...
If the chamber doesn't have a near mirror finish on it, it doesn't leave my shop!
If for no other reason it's to ensure a ROUND chamber and facilitate cleaning.

As for case 'Grip', what exactly do you think the SHEER LOAD of half soft cartridge brass is anyway?!
The SHEER LOAD is the pressure required to separate the brass from itself.
To scale off the outside micron or three, only where the brass finds purchase against chamber walls, wouldn't take 60k.

60k is simply going to sheer off the outside layer of brass and the case body will be on its way...
*IF* the 'Roughing' of a chamber were required to 'Hold' a case then the case would come out with the imprint of that roughing on the case.
It doesn't.
In fact, unless you have something seriously wrong with the chamber, like a seriously deep groove or gouge, the case comes out smooth as a baby's butt.
That alone should tell you the case isn't seeing any serious 'Sticking' to the chamber walls...
Brass galls VERY easily, stuffing it into a sizing die usually does more to the finish of the case than firing it does.

Again, I'll give an EXTREME example,
A fluted chamber like HK runs.
The flutes are to allow water to blow out of the barrel/chamber without blowing up the action, but the case comes out really dirty, but the case doesn't get formed to the flutes.
The case comes out round and is resizeable with common dies, this wouldn't be so if the case were swollen by firing pressure to hold it in place, the case would most certainly be formed to the flutes.

The next flaw I see is "60k in all directions",
The ONLY place in the case that 'Sees' 60k is the head during case recoil, directly backwards.
The case walls themselves have pretty good resistance to internal pressure, the round shape is ideally suited to holding back pressure...

Again, an EXTREME example,
A pressure burst disk.
Brass MUCH THINNER than the case walls is used to hold back 3,500 PSI in pressure tanks.
The purpose of the UNSUPPORTED disk is to blow out before pressure in the tank can rise above failure point of the tank walls.
The disk ruptures, releasing pressure in a controlled manner instead of the pressure vessel failing (exploding).
The pressure burst disks I use are not only a fraction of the thickness of a cartridge wall, but they are intentionally scored even thinner with a 'X' in the center.
About 1/2" of that disk is unsupported, with a 'X' scored into it making it even more prone to failure, and t will hold back about 3,500 PSI.

Granted, that's a long way from 50k-60k, but your cases are several times thicker and not weakened with a scored 'X' making them several times stronger...
Believing the case to chamber wall is seeing 50k-60k is a mistake in thinking.
The INSIDE of the case is seeing 50k-60k, but the case to chamber isn't seeing anywhere near that pressure.

The ONLY part of the case that 'Sees' the full chamber pressure is the head of the case, when the bullet moves and leaves a big, open hole in the mouth of the case.
That WOULD be 50-60k directly backwards, 50-60k directly opposed between back of bullet and case head. Opposite & Equal reaction to the bullet being forced down the barrel...

There have been 'Triangle' and 'Square' cartridge cases before, and they all failed when compared to a round case, splitting at the 'Corners' (tight bends).
Hotchkiss tried (more or less) square rounds because they stack tighter and feed smoother, more controlled, it was a total failure because the cases simply couldn't hold together at the same pressures/volume a round case can.

Like I said, believe what you want! It's not up to me to tell you what to 'Believe'.
The simplistic explanation that has become dogma, repeated & regurgitated adnauseam, just doesn't line up with the physics...
 
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