Primer protrusion

[Old Gunner]

Well its always been said that primer protrusion of a fired cartridge case is caused by insufficient pressure to push the case head back and reseat the primer.
the normal order of events being.

1. pin strike pushes case forwards
2. Primer detonation pushes primer back to limit of head gap clearance
3. expanded case grips chamber wall
4. chamber pressure pushes casehead back to contact breechface ,primer being reseated to full depth in pocket.

If the case grips the chamber wall tightly enough the case stretches.

With low chamber pressure

1. pin strike pushes case forwards
2. Primer detonation pushes primer back to limit of head gap clearance
3. expanded case grips chamber wall
4 Chamber pressure can not overcome friction fit of primer in pocket, or possibly residual pressure in pocket prevents reseating.

I can see the last scenario applying if the load is a squib or gallery practice load, but it takes very little pressure to seat a primer to begin with and I can't see this applying when chamber pressures exceed 20,000 CUP.
The expanded case walls on their own can't hold the entire pressure load preventing the case head from moving back, and at any higher pressure the case itself would stretch if the grip on the chamber walls was too great to allow it to slide back a few thousandths of an inch.

Bear with me.
I have a feeling that what happens is more like this.
1. pin strike pushes case forwards
2. Primer detonation pushes primer back to limit of head gap clearance
3. expanded case grips chamber wall
4. chamber pressure pushes casehead back to contact breechface ,primer being reseated to full depth in pocket.
5. residual pressure has yet to bleed out through the flash hole, only a matter of a few thousandths of a second at most.
6. Case if not stretched enough to suffer plastic deformation contracts to the limitation of it ability to recover from elastic deformation.
7. primer stays tight against breechface
Result primer protrusion under normal, if less than max, pressures.

Contributing factor would be the momentary, and very slight, expansion of the primer pocket under pressure.
Since permanent expansion of the primer pocket can occur through cold flowing at aproximately 70,000 + PSI for most cartridge brass, I figure temporary deformation can occure at much lower pressures. The last being why older guns sometimes show concentric gas cutting around the firing pin hole exactly where the perimeter of the primer pocket would be.

What do you guys think of this theory?

 

[Donald]

Short and sweet. I think you have excessive head space.

 

[pacecil]

Same old problem, as with most of the posts in this forum! Everybody is always coming up with good theories, but until you put some numbers with the theory you are kinda in the dark.

So, Old Gunner here's what you got: Lets assume the brass has a modulus of elasticity of around 14.000.000 psi and a yield strength of about 30.000 psi. Then let's assume the case is a 6PPC and about 1/2 " of case length will be stretched back toward the bolt face. This is probably too large a number and more than likely it is less than this - I have a feeling the stretching might take place over only about 1/8" of case. 1/2' can only be stretched about .001" before it reaches the yield point. So, even if you had .010 "head space" clearance you will only recover the elastic part of that stretch which would be about .001" That is, you would only see .001" primer protrusion regardless of how much head space you had. If the stretch was only 1/8" then you could have only about .0002" protrusion.

So, I think you are right about what occurs. I'm just trying to show you to what extent it might occur.

 

[Gunner223]

Old, My experience is that a relativly straight case will not fireform to the chamber and completely fill up the available head space. Thus the reason for altered brass when fireforming. If you use a RCBS case measuring gage I suspect that most of the "straight" (.223, .243 etc) cases will be about the same length before and after firing. This could be the reason most BR shooters prefer a very minimum length chamber. Mine are .005 under the go gage and I get no primer protrusion.

Don Carper

 

[John Kielly]

I do recall Parker Ackley's experiments with a 94 Winchester in .30/30 AI chambering which must have been running around 40,000 psi or thereabouts. He demonstrated & photographed that primers stayed proud of the case head when excess headspace was dialled into the rig & case & chamber were dry & clean - right up to when he really pushed the envelope & gave the rig more headspace than the depth of the primer, which ejected.

Does it have application here?

 

[SGS]

Primer Protusion

In your post you state: "The expanded case walls on their own can't hold the entire pressure load preventing the case head from moving back, and at any higher pressure the case itself would stretch if the grip on the chamber walls was too great to allow it to slide back a few thousandths of an inch."

Why do you think that the case can't grip the chamber wall enough to prevent the case from moving backward? This is not theory, but has been proven many times.If the chamber pressure stays below the yield strength of the brass, the primer backs out to the breechface and stays there. This occurrence is rare with the 6 ppc because the typical pressures are several times the yield strength of the brass, but is fairly common with lower pressure cartridges like the .35 Remington.

Scott Roeder

 

[Old Gunner]

I believe that in experiments like those of Ackley, and a recent attempt to duplicate it, that the observer is deceived by appearances.

I can see case walls gripping the chamber wall enough to result in the body of the case absorbing a fair amount of the back thrust on the bolt, that happens when dry chambers were used in British end crusher pressure guns, but as in those cases the reduction was far less than half the total back thrust and the case head definitely made full breechface contact and compressed the alloy cylinders. Pressures of the .303 were only a bit higher than the circa 42,000 PSI quoted for the .30-30 cartridge, case shape being very similar, and surface area in contact with the chamber wall being much greater.

I believe that the similarity in appearance of the primer protusion of a gallery practice level load, where impetus of the primer exceeds the max pressure of the load itself, and that same appearance when the load used is of far greater pressure (but not high enough to induce permanent plastic deformation) gave Ackley the wrong impression of what was going on inside the chamber.


If an unsupported case could hold the full force of a full charge, even that of a mild cartridge like the .30-30, then locking lugs would be superfluvious, and a .30-30 could be fired in a zip gun.

Ackley's experiment included removing locking lugs but the Browning/Winchester design has a high degree of idiot proofing and the cycling lever itself can prevent the bolt from being blown back.

Also to be honest I never gave Ackley's claims much credence, some claims attributed to him seem to be retellings of incidents described in Hatcher's Notebook with Ackley's spin added.

Over the years, and more often recently, I've run across a few people that will claim to have performed a test of one type or another and recognized that they are using the exact same wording that I'd seen used long before they were born.
Not saying that all gunwriters are like that, but some are about as trustworthy as a used car salesman at best.

Ackley's explanation did not take into account the elasticity of the case or the duration of the pressure cycle and residual pressures.
He thought in three dimensions but left out the fourth dimension "duration".

PS
Ackley's claim, if indeed he meant what his test results have been interpreted to mean, could be tested using the British type end crushser pressure gun and comparasion between oiled and dry chamber using the .30-30 cartridge in the same way the comparasion was made using the .303 cartridge. If the case took all the chamber pressure simply by clinging to dry chamber walls then there would be no pressure left to compress the alloy disc.

 

[SGS]

Ackley's claim, if indeed he meant what his test results have been interpreted to mean, could be tested using the British type end crushser pressure gun and comparasion between oiled and dry chamber using the .30-30 cartridge in the same way the comparasion was made using the .303 cartridge. If the case took all the chamber pressure simply by clinging to dry chamber walls then there would be no pressure left to compress the alloy disc.

I would like to see that. I would suggest starting with true low pressure loads of about 20,000 psi and working up to see what happens. 42,000 is a relatively low pressure load by today's standards, but still probably above the yield strength of the brass. I would also consider using other cartridges including straight wall and rimfire.

The results are probably moot in today's world, but It would be interesting to remove any doubt as to what actually happens.

Scott Roeder

 

[Old Gunner]

I would like to see that. I would suggest starting with true low pressure loads of about 20,000 psi and working up to see what happens. 42,000 is a relatively low pressure load by today's standards, but still probably above the yield strength of the brass. I would also consider using other cartridges including straight wall and rimfire.

The results are probably moot in today's world, but It would be interesting to remove any doubt as to what actually happens.

Scott Roeder

I just thought of a simpler method of testing my theory. But unfortunately I don't own a .30-30 and don't know anyone who does.


I recently ran across a post somewhere in which the poster mentioned using a material that I'd seen advertised years ago as a method of getting a chamber pressure reading when testing handloads.

If I remember correctly its a thin plastic wire inserted between case head and breechface. After firing the material is miked to measure how much it compressed. Its been many years since I read about this so I don't really remember the details of how it worked, if indeed it did work as designed.

Simply proving that the casehead moves back to the breechface could be accomplished using a thin lead wire or strip. If it shows any compression at all after firing then the case can not have been effectively locked to the chamber wall by internal pressure.

Perhaps a piece of solid solder or lead wire curled into an O and placed so the primer could contact it. The wire would have to be tapped down flat beforehand and be only as thick as the head gap clearance.
The headstamp lettering should leave a negative impression in the lead.

I will be doing some experimenting with light loads in the .303 this summer, but the claim was never made in relation to any cartridge of that class. Otherthan the aproximate 1/3 reduction in back thrust recorded using a dry chamber and casing in the pressure gun as opposed to the oiled chamber and cartridge used for maximum thrust.

The difference in the pressure gun readings don't mean theres an actual difference in the chamber pressure, its just a matter of how much is applied to the breechface.
Sort of like the difference between actual recoil and perceived recoil to a shooters shoulder if a pad is used.

 

[alinwa]

JEEEpers, would you'se guys TEST THIS STUFF before spouting theories??

Plasti-gage costs nearly NOTHING and yes I've tested this on many cartridges.

WITH EXCESS HEADSPACE, the primer protrudes by roughly the amount of play present until the brass "yields" or stretches right at the web/body junction. If the brass doesn't yield on the first firing the headspace can stay long for several firings. When shooting necksize-only it's possible to watch your primer protrusion lessen with each firing........ if you've got the proper tooling you can also watch the casehead cant around as it works its way back.

Once pressure gets high enough to stretch the case in one whack, it does, and hammers the primer back home. Those primers that are flattened and mushroomed across the primer cup chamfer are the result. ONLY way you can get 'em....... The resultant thin spot, the occurrence of head separation and the grainy, shiny ring of brass indicating incipient head separation are all documented FACT. You want PICTURES??? Buy a newer Nosler reloading manual. They intimately photograph the whole sequence.

You wanna' SEE IT?? Go test it yourownself....... Screw your FL die down to reset the shoulder by 5thou and fire it until it blows.....4-5 reloads should do it.

EVERY "normal" case acts this way, exceptions are the Super Short Mags which have a whole nuther set of (related) problems.

In My Opinion there are many cases (and the entire Lapua line) which will run up to around 50,000psi without yielding. My SWAG is based only on observation and velocity over a chronograph, and interpolation. The starting loads in most reloading manuals will not stretch Lapua or Winchester brass.

Historically hundreds of thousands of Palma Competition loads would "teeter on the table" so that you could measure the headspace.

YES a 30-30 will stick and grip the chamberwalls, as will even a tapered antique like the 22-250. The tapered cases will also tend to stick the boly on primary extraction but this is another issue. And there are some cases like the ubiquitous 30-06 which have such a shallow shoulder angle and short shoulder that the headspace can be reset by the firing pin! Given new and fragile brass and a loose fit, and one of the "upgrade" springs and pins. Again, documented tested fact. Research machine gun design to see where he 30-06 round cannot be adapted to many systems because the act of chambering a round drives the case in hard enough to reset the headspace.


To all's of you who want to apply book figgers and guesses to the thang ....... just reset the shoulder on some new cases, MAKE about .005 or so MEASURED headspace and do a load workup. WATCH the primer pop up........This tends to add perspective.

Note that there is NO RE-SEATING MECHANISM for the primer, the cartridge does grip the chamber walls, it NEVER slides back.

Plasti-gage it.......

I'm eager to hear of some testing.

al

 

[Boyd Allen]

Many years ago, I went to the range with a Springfield that had a very heavy striker spring, and a fresh military barrel, parkerized. I mention all of this to set the stage for an experiment that I did. I also took with me a Lee loader a micrometer powder measure, that I had calibrated, some 748, and some 150 grain bullets, along with brass, a priming tool, some primers, and a plastic hammer.

Previously I had determined the measure setting that would give me a slightly below middle of the book load as well as the setting that would give the maximum.

When I got to the range, I fired the same set of five cases several times. After each firing the primer protruded a little farther from the case head. I believe that this was because the load did not make enough pressure to take the brass back near the case head stretch past its elastic limit.

The heavy firing pin assembly, and strong spring had combined with the rather small shoulder angle so that the case was knocked a little farther forward in the chamber each time it was fired. This combined with the long body dimension and rough chamber to function as a sort of ratchet, if you will.

Being an adventuresome (foolish) sort, after the primers were protruding a ridiculous looking amount, I loaded the max load, and fired the cases again. The primers were flush, and there was a bright line just ahead of the case head, indicating an "incipient separation", at which point I smashed the necks flat with a pair of pliers, so that no brass scrounger would reload them, and threw them away.

In my experience, proper attention is paid to shoulder bump, there should hardly ever be a problem of this sort with full power loads. On the other hand, moderate jacketed bullet, or low pressure cast bullet loads to merit special vigilance. One should keep and example case that has been fired with a load that is hot enough so that the primer is flush, as a reference for that rifle and barrel, and compare its head to shoulder datum line measurement with fired cases and when setting up shoulder bump.

As to the question of the exact mechanism of these events, I leave you to your own conclusions, based on your own testing, or the data that I have just related.

 

[alinwa]

Being an adventuresome (foolish) sort, after the primers were protruding a ridiculous looking amount, I loaded the max load, and fired the cases again. The primers were flush, and there was a bright line just ahead of the case head, indicating an "incipient separation", at which point I smashed the necks flat with a pair of pliers, so that no brass scrounger would reload them, and threw them away.

"HEY, Try Another Lever Kronk!!"

LOL




".... where angels fear to tread eh???"

Ain't we all..



al

 

[Boyd Allen]

Hey Al,

Which side are we shooting from these days?

 

[alinwa]

Hey Al,

Which side are we shooting from these days?

right

al

 

[Old Gunner]

JEEEpers, would you'se guys TEST THIS STUFF before spouting theories??

I don't see that theres been any "spouting" of theories, presenting a theory based on observations and asking for input is the way to find out how things work. No one can look into the chamber to observe this directly, not without multi million dollar equipment anyway.

Plasti-gage costs nearly NOTHING and yes I've tested this on many cartridges.

I've never heard that this plasi-gauge was a particularly accurate way of measuring chamber pressure so I never tried it. I've never seen it in gunshops around here, if I had I'd have obtained some just to try it out.
I think you'd agree that the case head does come back in any load at reasonable pressures otherwise the plasti gauge would be useless.

WITH EXCESS HEADSPACE, the primer protrudes by roughly the amount of play present until the brass "yields" or stretches right at the web/body junction. If the brass doesn't yield on the first firing the headspace can stay long for several firings. When shooting necksize-only it's possible to watch your primer protrusion lessen with each firing........ if you've got the proper tooling you can also watch the casehead cant around as it works its way back.

Once pressure gets high enough to stretch the case in one whack, it does, and hammers the primer back home. Those primers that are flattened and mushroomed across the primer cup chamfer are the result. ONLY way you can get 'em....... The resultant thin spot, the occurrence of head separation and the grainy, shiny ring of brass indicating incipient head separation are all documented FACT. You want PICTURES??? Buy a newer Nosler reloading manual. They intimately photograph the whole sequence.

You wanna' SEE IT?? Go test it yourownself....... Screw your FL die down to reset the shoulder by 5thou and fire it until it blows.....4-5 reloads should do it.

Ive limited my handloading to rimmed cartridges for many years, I handloaded for the 7.92X57 when I first began shooting.
The results of trying lower pressure loads with faster powders did get me to thinking on what goes on in the chamber at that time.

My handloading of the .303 has been full power only.
One of my .303 rifles had generous headspace when I got it, and I'm familar with stretched cases and case cracking. I replaced both the bolt and bolthead of that rifle and no longer have that problem.
Never saw a protuding primer due to the excess headspace of that rifle, or afterwards when headspace was corrected to less than .004 clearance.

Excessive headspace situations aren't exactly what I'm speaking of, that just makes the situation more obvious to the eye.

EVERY "normal" case acts this way, exceptions are the Super Short Mags which have a whole nuther set of (related) problems.

In My Opinion there are many cases (and the entire Lapua line) which will run up to around 50,000psi without yielding. My SWAG is based only on observation and velocity over a chronograph, and interpolation. The starting loads in most reloading manuals will not stretch Lapua or Winchester brass.

Historically hundreds of thousands of Palma Competition loads would "teeter on the table" so that you could measure the headspace.

YES a 30-30 will stick and grip the chamberwalls, as will even a tapered antique like the 22-250. The tapered cases will also tend to stick the boly on primary extraction but this is another issue. And there are some cases like the ubiquitous 30-06 which have such a shallow shoulder angle and short shoulder that the headspace can be reset by the firing pin! Given new and fragile brass and a loose fit, and one of the "upgrade" springs and pins. Again, documented tested fact. Research machine gun design to see where he 30-06 round cannot be adapted to many systems because the act of chambering a round drives the case in hard enough to reset the headspace.


To all's of you who want to apply book figgers and guesses to the thang ....... just reset the shoulder on some new cases, MAKE about .005 or so MEASURED headspace and do a load workup. WATCH the primer pop up........This tends to add perspective.

Note that there is NO RE-SEATING MECHANISM for the primer, the cartridge does grip the chamber walls, it NEVER slides back.

Plasti-gage it.......

I'm eager to hear of some testing.

al

Depending on the chambering many cases do slide back under full pressure, otherwise blowback operating systems could not work at all, the case acting as the piston.
Higher powered blowback systems (Austrian Schwartzloss, and a few others) used either lubricated cases or gas cushions to prevent the case from clinging and the case head being torn off.

I know that the case clings to the chamber wall if both are dry, otherwise annular rings would not happen.

Going back to Ackley's experiment he figured the case didn't move at all, I'm saying that the case would stretch but normally not beyond its ability to recover and spring back if pressures were in the 38,000 Cup/42,000 psi range for the .30-30 as quoted.

following may not be pertinent but somethings I've noted.
When loading for my Mauser I used 4198 and 4227 reduced charges (around 32-35 grains, forget exactly) that gave velocities close to .30-30 levels. The cases did get progressively shorter from base to shoulder. But primers were occasionally flattened flush with the base anyway, sometimes flattened as if from excessive pressures though the case showed no signs of even normal pressures. Not sure why the primers would flatten under those conditions. Cases were from Canadian ammo meant for the BESA Tank and aircraft MG , very tough brass.

When loading wax bullet and cat sneeze loads for the .38 special I found that I had to drill out the flashole when using primer power only to drive wax bullets, otherwise the primer protuded enough to cause cylinder drag.
cat sneeze loads of one grain of bullseye and a round ball gave no such problem. Pressures were not high enough to grip the chamberwalls even with full loads of 3.5 grains Bullseye under a 158 SWC.

I've always been a seat of the pants reloader, but have avoided those "hold my beer and watch this" moments.

To be clear I'm saying that unless the chamber is free from any oil the case will usually slide back under pressure, and if chamber and case are dry the case will stretch under pressure, these I believe we can agree on.
What I feel may not be visible to the eye is the extent that a stretched case can snap back (elasticity) if pressure does not induce plastic deformation, and whether the primer would necessarily remain flush as the case freed from pressure snaps back.
If pressures exactly match the elasticity of the case then a well balanced load would allow absolute maximum case life without noticable change in the case dimensions.
Reloads would chamber as freely as loads using fresh cases.

The only rifles I'll be loading for anytime soon are chambered for rimmed cartridges, so cases won't suffer any shoulder set back ,the rim prevents that.
All have headspace well within factory specs.

The only way I see to test Ackley's claim is to use a .30-30 rifle. Its the only chambering that the claim has ever been made for.
If true his experiment would mean that the breech face served no other purpose than to support the primer cup.

You may be confused by my use of the term "mechanism".

PS
I checked a few sites, including precision barrel manufacturers, and I've found no source that claims that a cartridge case can absorb more than one half the back thrust on firing.
That seems to agree with the British pressure gun results.

 

[alinwa]

Old Gunner,

"Same Planet, Different Worlds"

Well, what you've demonstrated is that you and I think differently. We approach a problem differently and interpret the results differently.

No, I can't agree with the items that you "think we agree on." In simple terms I DO NOT AGREE with the stated items.

BTW, you keep referring the "snap back" and elasticity" of brass as though this is your own new idea and "explanation" for various phenomena. The thin casewall brass snaps back about a thou radially, you can measure it, test for it. Thick casehead brass doesn't snap back at all radially. Primer pockets do NOT open up and pop back..... you can test for it. Brass casewalls will snap back about a half-thou longitudinally, you can test for it.



As far as answering your post point by point....... I don't even know where to start man. Just WOW..... stuff like this just causes my brain to lock up! Depending on the chambering many cases do slide back under full pressure, otherwise blowback operating systems could not work at all, the case acting as the piston.
Higher powered blowback systems (Austrian Schwartzloss, and a few others) used either lubricated cases or gas cushions to prevent the case from clinging and the case head being torn off.



But if you "need a 30-30 to test Ackley's claim" I'll be happy to supply you with one. Or if you need some plasti-gage to test with, I can send you some... This is sometimes better than getting your information from internet sites or books because it allows you to read WHAT the site or book says instead of reading into what the site or book says. "Hands on" just makes stuff more real.....

al

 

[Old Gunner]

Old Gunner,

"Same Planet, Different Worlds"

Well, what you've demonstrated is that you and I think differently. We approach a problem differently and interpret the results differently.

No, I can't agree with the items that you "think we agree on." In simple terms I DO NOT AGREE with the stated items.

BTW, you keep referring the "snap back" and elasticity" of brass as though this is your own new idea and "explanation" for various phenomena. The thin casewall brass snaps back about a thou radially, you can measure it, test for it. Thick casehead brass doesn't snap back at all radially. Primer pockets do NOT open up and pop back..... you can test for it. Brass casewalls will snap back about a half-thou longitudinally, you can test for it.

I think it depends on the individual case design and alloy.
I think more is going on under that level of pressure than meets the eye.
Since pockets do swell permanently if pressure is high enough, there should be a point where they might open only a tiny amount and be able to spring back almost but not quite instantly.
Heat could contribute to this.

If you have a source detailing the amount of maximum elasticity of cartridge cases I'd like to see it.
The more information to work with the better.

As far as answering your post point by point....... I don't even know where to start man. Just WOW..... stuff like this just causes my brain to lock up! Depending on the chambering many cases do slide back under full pressure, otherwise blowback operating systems could not work at all, the case acting as the piston.
Higher powered blowback systems (Austrian Schwartzloss, and a few others) used either lubricated cases or gas cushions to prevent the case from clinging and the case head being torn off.

This sort of high pressure blowback operated autoloader is fairly well known, More efficient systems eclipsed these long ago.
There were experimental systems like the Evans .30 Carbine caliber handgun that used indentations in the chamberwall to provide a hesitation blowback.
The tapered .30 carbine case would not hold the entire force of the charge unsupported by a locked breech for very long even when locked mechanically into the indentations. Chamber pressure levels are close to that of the .30-30.

But if you "need a 30-30 to test Ackley's claim" I'll be happy to supply you with one. Or if you need some plasti-gage to test with, I can send you some... This is sometimes better than getting your information from internet sites or books because it allows you to read WHAT the site or book says instead of reading into what the site or book says. "Hands on" just makes stuff more real.....

al

I might take you up on it. Got too much on my plate right now though.

I've been examining a fired Federal .30-30 case I picked up at a shooting spot years ago.
The base shows visible evidence of contact with the breechface though the primer protudes about .004. The face of the primer is flattened and has markings from the breechface clearly visible.

I have a few more picked up from several widely separated spots, so presumably from a variety of .30-30 rifles. I'd noticed that must fired Winchester .30-30 factory loads usually show a slight protusion, one reason I kept those cases.

So far the only, and I mean only, type of weapon that can fire a high pressure cartridge without support from a breechface is the recoil less rifle. There the chamber exhausts gas through a venturi to counteract the back thrust. The more primitive form uses a charge of shot or other heavy material to provide equal resistence and take the place of a breech bolt.

Since the difference in the amount of back thrust between oiled and dry chambers is recorded scientifically as part of the British Proof testing system that information is certainly relevant.

If I ignore published accounts I can simply ignore Ackley's test results as well.
Afterall its just words on paper at the end of the day.

Claims like Ackley's go against conventional wisdom, not damning in itself, and the results of recorded scientific studies. So its either bull or theres another explanation for the perceived result.

PS
It occurs to me that the primer pocket must be capable of expanding and contracting a tiny amount in order to remain tight enough for reuse.
The primer necessarily expands in the pocket to form a seal, the brass of the casehead is not adamantine, it has to give. If it didn't recover then the case would not be reloadable. The amount of give there would be in tenths of thousandths or less.

According to this
http://yarchive.net/gun/ammo/cartridge_expansion.html
Brass may be more elastic than you think it is.
If the case can expand .004 radially and recover, then the much greater length of the material of the case body linearly should allow even more expansion.
The case walls are tapered in thickness as well, so the load isn't the same throughout its length.

Also when a cylindrical solid (case head) is compressed end to end it expands radially. The Primer still under high pressure might act like a mandrel to expand the pocket slightly.

 

[Old Gunner]

I'm going to add this as a separate post so if reproducing this here is a copyright violation the moderator can just delete it.
I'm going on the assumption that the portion of a much longer article can be posted under the "Fair Use" principle for educational purposes.

THE CASE
As the striker hits the primer, it drives the case forward in the chamber (to the extent that headspace will allow). On some cases with poor headspace control (e.g., 35 Whelen), the striker can drive the case into the chamber with enough force to move the shoulder back and thereby increase headspace.

As the primer pellet explodes, it generates considerable additional force that works to drive the case forward (and will do so, if headspace control is inadequate) and the primer backward (the primer moves until it is supported by the bolt). For cases using small primers, this force is about 750 pounds; for cases using large primers, this force is about 1,500 pounds (cases that have small-diameter flash holes generate greater force).

As the powder charge ignites and chamber pressure begins to build, the hollow portion of the case stretches, to fill the available space. When chamber pressure reaches approximately 3,000 psi, the case walls begin to push against the chamber. Thereafter, as pressure progressively increases, case-to-chamber bonding becomes progressively more solid.

When the force inside the case becomes sufficient, the case body yields and the case head begins to move rearward. The amount of pressure the case can withstand before the case head begins to move rearward depends upon case wall thickness and hardness (in the area near the body-to-web transition), difference between web diameter and primer pocket diameter, and how long pressure stays above the threshold pressure (where case begins to yield).

In most typical loads used in modern guns, chamber pressure will always be sufficient to drive the case head rearward and thereby stretch the case walls. Depending upon case wall thickness and degree of wall thickness taper near the body-to-web transition, the case can elastically stretch some amount — typically several thousandths inch. When this stretching exceeds the elastic limit for that case, the case walls will permanently thin and lengthen. This damage usually occurs about 0.1-inch forward of the floor of the case web.

After chamber pressure becomes sufficient to initiate case wall stretching, the case head soon hits the bolt. As pressure progressively increases, it pushes the case head progressively harder against the bolt. Hence, the bolt will progressively compress and the action will progressively stretch until chamber pressure peaks. (Actually, owing to inertia, the case head will continue to push against the bolt, as the bolt continues to retreat, for some time after chamber pressure has peaked.)

Eventually, the bolt will exert more force against the case head than chamber pressure exerts against the case head. At that instant, the bolt will begin to slow, eventually to stop, and then to reverse direction. As chamber pressure continues to plummet, the energy stored in the bolt and receiver will drive the bolt back toward the resting position. In practice, the bolt will hammer the case into the chamber with considerable force; often sufficient to set the case shoulder back enough to assure that case headspace length is shorter than chamber headspace length. As a result, the action will open freely.

While all of this is happening, the chamber also is stretching in both length and diameter. This contributes some to case stretching; it also supplies the energy that allows the chamber to hammer back against the case body, so that it is reduced in diameter enough to assure free extraction.

Depending upon case shape (body taper, shoulder width, shoulder angle), case construction (hardness, thickness, etc.), load variables (pressure peak and duration), action design and barrel design (over the chamber), many results are possible. First, the action can open freely and the case can extract freely. Second, the bolt can turn freely (because case headspace is shorter than chamber headspace) but the case can hang up in the chamber (because relaxed case body diameter is slightly larger than relaxed chamber diameter (the case is an interference fit). Third, the bolt can turn hard (because case headspace length is longer than chamber headspace length) but the case can extract easily (because case body diameter is smaller than chamber diameter).

This partly explains why various chamber and gun designs show different symptoms with loads at similar peak pressure. The classic comparison is the 22-250 versus the 22-250 Ackley Improved (AI). With the former, this rebounding bolt can easily drive the case into the chamber far enough to move the shoulder and solidly wedge the case; with the latter, the shoulder is many times more resistant to being moved and driving the case into the chamber the same distance accounts for many times less increase in case diameter at any given location. Hence, pressure that causes sticky extraction with the 22-250 will show perfectly free extraction in the AI version.

THE GUN
How much the case head moves the bolt face rearward depends upon the following factors:
Shape of pressure curve — a wider curve means more movement (owing to inertia, the case head never has time to move as far as it would if the same peak pressure were applied in a static situation. Therefore, the longer the pressure stays close to the peak, the more the bolt will compress — the farther the head will move);
Peak chamber pressure — force and movement are directly proportional;
Distance between bolt face and locking surface — distance and movement are directly proportional;
Cross-sectional area of bolt — area and movement are inversely proportional; and,
Cross-sectional area of receiver — area and movement are inversely proportional.

http://www.longrangehunting.com/articles/happens-fire-gun-1.php

From
What Happens When You Fire A Gun?
By M.L. McPherson
©Copyright 2008, The Varmint Hunters Association, Inc.

The following site
http://www.varmintal.com/a243zold.htm
Has computer simulations of various states of chamber finish and friction coefficients. In no situation does the cartridge case head not come into contact with the breech face, even if the chamber is rough as a cob, the case either slides slighty or stretches.

WHAT IS THE FRICTION COEFFICIENT?.... The friction coefficient is a dimensionless number and is the ratio of the force to cause sliding divided by the normal force the object applies to the sliding surface. Imagine a block of wood setting on a table. If the coefficient of friction between the block of wood and the table surface is u = 0.6, for example, then if you push down with a 10 pound force on the block of wood, it would take a 6 pound force parallel to the table's surface to make the block slide across the table.

The above site did not factor in all possibilities concerning the primer and pressures inside the primer pocket, I don't think there is that much detailed information on the primer and its pocket compared to the other elements involved.

No single source has all the information I'd need to prove my own theory on why the primer protudes to the limit of headspace when the loading is mild but pressures are high enough that the case head moves against the breechface.
I can't see that chamber pressure could overcome the entire force of back thrust as Ackley's experiment suggested.

The pressure curve and the fact that there are known demonstrated conditions where the pressure is higher at the shoulder than at the base lead me to belive that most of the gripping action takes place near the shoulder and neck, leaving much less grip to overcome further back, allowing the case wall to stretch.

The gif towards the bottom of the page is of an annealed casehead with plastic deformation. I think that elastic deformation of the primer pocket would follow much the same pattern, but be so slight as to be almost imperceptable and last for only milliseconds.

 

[alinwa]

Please read and understand "Rifle Accuracy Facts" by Harold Vaughn.

Please understand that Ol' Mic McPherson ain't held in as high regard here as over on the other forums.

Please understand that while Varmint Al spends a lot of time here, his work isn't automatically accepted as meaningful either. He's learning though that experimental data DO NOT always agree with knee-jerk computer analyses...... that often there are factors involved that the experimenter didn't factor or wasn't aware of.

Please take the time to understand stiction. I wish Varmint Al and certain others would! There are a whole bunch of people gabbling endlessly about chamber polish and finish while dragging brass blocks over steel plates to establish "friction coefficient"...... meaningless BS.

PLEASE just DO some of this stuff!!!! A whole BUNCH of your ideas are hopelessly flawed and believe me, when you make statements like "Depending on the chambering many cases do slide back under full pressure, otherwise ........" to illustrate the actions of a modern high pressure necked cartridge like the 6.63 NATO round you're not doing your case any good.

Around here people DO HEAR YOU and DO UNDERSTAND YOU and aren't easily baffled by BS.

The broad statements contained in ML McPherson-type articles work well for consumption by the masses but around here you need to do better. This ain't your typical glossy-mag reading bunch. For every one of your tests. assertions and ideas there are 50 guys here who've DONE IT before, TESTED it.......

F'rinstance:

You wanna' KNOW how much springback the various brass compositions exhibit under firing conditions??? Then TEST IT! Many of us have and do on a daily basis. I deal with springback every day. I HAVE to know my springback to shoot at the level I shoot, no guessing or speculation. I've had reamers made to match chambers and dies from scratch and been able to mix/match loaded rounds. (I'm guessing that this means nothing to you, but to do this requires an intimate knowledge of springback characteristics.)

And keep up with the head-modeling but don't STOP when you think you've got it! This "I think" stuff needs to be just the beginning! Now go TRY IT......

BTW, your thing about the rings on the bf being from gas leakage??? You're RIGHT, but only partly so. The rings are from gas-cutting (A well known phenomenon) but the mechanism is NOT that the sphincter farts a little and reseals leaving no trace......... the gas cutting occurs when the primers "leak" (search it to see that BR guys are very familiar with the concept!) but it's far from imperceptible as you need only look at the primer to spot the black sooty deposit. Also, the pockets do not collapse back, in fact loose primers are the single most common way of diagnosing high pressure. I run EVERYTHING up to this point of primer leakage. I build my rifles from the ground up to maximize this point. Bottom line is this, once a primer leaks IT'S OVER for that case. Folks do continue to seal them using everything from fingernail polish to primer sealant to cigarette paper to super glue but the primer pockets do not ever spring back.


Soooo, your views are welcome indeed, , but I hope you're not dismayed by the fact that folks here deal in FACTS not idle speculation. We build and SHOOT this stuff. I remember Varmint Al leaving this board for nearly a year over some of his assertions VS the FACT that people were doing what he said was impossible....

Now that I think of it....... it's happened more than once. I haven't seen him for a while! Maybe he'll get in on this...... I love the way he thinks and exposure to extreme accuracy (and extreme pressure!) have changed his views over time. His work with Esten and tuners has been fairly linear and fruitful although I still disagree with some of the presented hypotheses. He interprets data sometimes instead of exegetically accepting it and analyzing it.

And some of McPherson's stuff borders on the ludicrous. ML hares off on some truly goofy tangents and makes some assertions that cannot be demonstrated experimentally nor supported theoretically. Search "TPT" to experience some of McPherson's "logic."

So anyways..... no excuses! If you're gonna' theorize, show us some WORK!

LOL

get 'er done.......

al

 

[Old Gunner]

PLEASE just DO some of this stuff!!!! A whole BUNCH of your ideas are hopelessly flawed and believe me, when you make statements like "Depending on the chambering many cases do slide back under full pressure, otherwise ........" to illustrate the actions of a modern high pressure necked cartridge like the 6.63 NATO round you're not doing your case any good.

Is "6.63 NATO" a typo?
The subject was cartridges that exhibit primer protusion when pressures should be adequate to force the casehead back to the breechface, the only serious testing on this I've seen quoted is Ackley's test of the .30-30.
If you are getting this sort of protrusion with other cartridges I'd like to hear more about that.

In case you lost track of what I'd written earlier.

Depending on the chambering many cases do slide back under full pressure, otherwise blowback operating systems could not work at all, the case acting as the piston.
Higher powered blowback systems (Austrian Schwartzloss, and a few others) used either lubricated cases or gas cushions to prevent the case from clinging and the case head being torn off.

I know that the case clings to the chamber wall if both are dry, otherwise annular rings would not happen.

Going back to Ackley's experiment he figured the case didn't move at all, I'm saying that the case would stretch but normally not beyond its ability to recover and spring back if pressures were in the 38,000 Cup/42,000 psi range for the .30-30 as quoted.

And you can't deny that blowback systems do work, often at pressures higher than that of the .30-30.
Either the case slides back or it stretches, one or the other. It doesn't just sit there absorbing the entire force of the charge without effect on the brass.

The fact that oiled cartridges are used along with dry chambers and cartridges to establish the difference in back thrust on the boltface and that every published source on these shows that a dry chamber reduces back thrust by 1/3 to 1/2 but never eliminates it goes counter to the claim that chamber pressure acting on the walls can lock a case imovably without the case stretching back to meet the bolt face.
Thats an established fact of a century or more of testing cartridge pressures for the British Military and sporting rifles.

The spring back effect of locking lugs and its effect on the cartridge case was established a century ago as well. I can't see that there has been any modern improvement that would eliminate that.
Appearances can be deceiving.

PS

BTW, your thing about the rings on the bf being from gas leakage??? You're RIGHT, but only partly so. The rings are from gas-cutting (A well known phenomenon) but the mechanism is NOT that the sphincter farts a little and reseals leaving no trace......... the gas cutting occurs when the primers "leak" (search it to see that BR guys are very familiar with the concept!) but it's far from imperceptible as you need only look at the primer to spot the black sooty deposit. Also, the pockets do not collapse back, in fact loose primers are the single most common way of diagnosing high pressure. I run EVERYTHING up to this point of primer leakage. I build my rifles from the ground up to maximize this point. Bottom line is this, once a primer leaks IT'S OVER for that case. Folks do continue to seal them using everything from fingernail polish to primer sealant to cigarette paper to super glue but the primer pockets do not ever spring back.

We apparently aren't thinking of the same sort of ringing on the breechface. My observations have been of dozens of military rifles usually only recently finding their way to the civilian market ,presumably never having been fired with reloaded ammunition, only military ammunition designed for single use, and in most cases with staked primers.
The ringing is very light and even with no sign of a cutting action or jet of gas from an imperfection of a single primer or pocket.
The appearance is that of a cummulative action of very slight leakage at the lower pressure levels rather than at peak pressure. The marking of the breechface is the result of a very slight leakage of many thousands of rounds fired, probably tens of thousands by the look of those rifles with the most noticable rings.
The appearance is usually attributed to corrosive primer gases being forced into the metal.

So unless you build a rifle using the same steels used in the early 20th century and then fire 30,000-50,000 rounds of corrosive primed military ammo through it you aren't going to be able to duplicate the sort of ring I'm speaking of.
Checking the progress of the ring's appearance every few years as you go of course.
I've noticed the same sort of rings on the breech faces of 19th century breech loaders as well, they'd also have used corrosive primers, no sign of gas cutting there either, just the surface marring and corrosion pitting of cummulative leakage at the lower ends of the pressure curve.

Some things we can observe can't be easily repeatable as an experiment, life is too short.