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Thread: Primer protrusion

  1. #1
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    Primer protrusion

    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?

  2. #2
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    Short and sweet. I think you have excessive head space.

  3. #3
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    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.

  4. #4
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    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

  5. #5
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    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?

  6. #6
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    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

  7. #7
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    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.
    Last edited by Old Gunner; 06-04-2009 at 03:15 AM.

  8. #8
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    Quote Originally Posted by Old Gunner View Post
    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

  9. #9
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    Quote Originally Posted by SGS View Post
    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.

  10. #10
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    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

  11. #11
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    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.

  12. #12
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    Quote Originally Posted by Boyd Allen View Post


    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

  13. #13
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    Hey Al,

    Which side are we shooting from these days?

  14. #14
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    Quote Originally Posted by Boyd Allen View Post
    Hey Al,

    Which side are we shooting from these days?
    right

    al

  15. #15
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    Quote Originally Posted by alinwa View Post
    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.
    Last edited by Old Gunner; 06-06-2009 at 08:34 AM.

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