What’s the science behind loose primer pockets?

[Mike Casselton]

Not asking about certain brass. Asking what it is in firing that is actually causing this event to happen and how it happens.

Measure the diameter of a primer that's been fired in a case with too much pressure. You'll see that it's diameter is larger than an unfired primer.

What happens is that you have too much pressure.

In a normal sense, the primer is pressed into the pocket and pretty much seals the back of the brass.
The primer is sitting against the bolt face and it's prevented from moving rearward.

With normal pressure, the case extracts easily, and the primer is still round on the edges.


If you hammer it with very high pressure, the pressure expands in all directions.
One of those directions is rearward.
This high pressure goes back through the flash hole and tries to expand the now empty primer shell.
Since it can't expand rearward, the pressure goes to the least path of resistance which is radial.
This expands the primer shell, its rear radius and then if the pressure hasn't dropped enough, then the pocket expands radially.

Anyone that's played with blowing up balloons as a kid should be able to visualize this.

So yeah, it's too much pressure.

Here's some 17 Hornet that have been shot over pressure. Look at how large the primers have grown.

 

[tomme boy]

How do you know this?

Head ballistician from Ruger. He has been a internet friend of mine for years. I trust what he says with out a doubt.

 

[Yondering]

Some brass "federal" comes very soft to begin with. The head is not struck one last time to harden it. It is a cost cutting measure that others are going to also. So primer pockets can become loose on the very first firing on a safe load.

I have experienced what I believe are symptoms of this in a couple batches of Federal 223 brass, using loads that are not overly hot (like 23.5gr of 8208 behind a 75 BTHP, a known accuracy load). Since velocity was nearly identical to that from other Fed brass that didn’t have an issue, I’m pretty sure the batch of cases that blew the primers out were simply too soft at the head.

I also experienced something similar with 6.8 SPC Nosler brass years ago, where primer pockets were loosening with loads at and below min starting charges, and corresponding low velocity. IME the 6.8 is a bit more prone to this anyway, but that whole batch of Nosler brass seemed to have soft case heads. Too bad, cause they weren’t cheap.

In either case, if a striking operation was skipped for cost savings on that brass, it’d explain a lot. And it wouldn’t be the first time an ammo component manufacturer cut corners. Brittle Obama-era Winchester large rifle primers for example…

 

[308pirate]

Head ballistician from Ruger. He has been a internet friend of mine for years. I trust what he says with out a doubt.

Federal has always been soft. What's your buddy's source that other companies are making their brass softer? Did they tell him? Does he test the brass for hardness?

Just being a ballistician doesn't mean that he has access to what is almost certainly proprietary manufacturing knowledge from companies he doesn't work for.

 

[918v]

Federal makes two kinds of brass. Their military brass is harder than their commercial brass with the exception of 300WM which is soft in both applications.

 

[tomme boy]

Federal has always been soft. What's your buddy's source that other companies are making their brass softer? Did they tell him? Does he test the brass for hardness?

Just being a ballistician doesn't mean that he has access to what is almost certainly proprietary manufacturing knowledge from companies he doesn't work for.

It does when he calls and they tell him why. This came from firing thousands of rifles with factory loadings that all had loose primer pockets after the first firing. And pressure testing the rounds and see they are within spec. but for some reason the brass is junk. You would want to know why.

The other companies it is my guess as the amount of trash brass I have picked up that had loose pockets from the original firing.

 

[tomme boy]

And another brass to be warry of is starline 350 Legend brass. As was shown in the earlier post about low web heights some starline brass has really thin webs. So you can turn your regular brass into a belted mag case. Or blow it out completely. I have not checked any of their other brass if it has low webs or not.

The firearms you have to be careful of this are anything based off the AR15 firearm that has a coned breech face.

 

[bax]

I don't understand the dynamics here. I accept that the barrel/chamber gets larger from pressure and that the brass is transmitting the pressure from the inside of the case to the steel. The powder makes 50k PSI, forces the brass against the chamber walls, the steel deforms elastically and the brass is along for the ride. The deformation exceeds the brass yield strength and the brass deforms plastically. Got it.

So I accept that the brass experiences plastic deformation - this explains why we have to resize our brass. I accept that full-length sizing reduces the size of the body - another (opposite) plastic deformation. I do not accept that full length sizing reduces the diameter of the case head. I have too many pieces of 300 win mag where the case had stopped fitting into the shell holder.

I can't figure out why the primer pockets get bigger. Why don't they get smaller? If the pressure is pushing on the the brass in the base, the base could get thinner and wider. That is, the base of the case would fit the chamber/bolt face more tightly because the diameter is larger but the distance from the bolt face to the top of the flash hole is less. If the brass is compressed, why did the primer pocket get bigger? Where did that brass go?

Maybe the case head just gets wider? Why would it compress in only one dimension? Some pressure is pushing toward the bolt face parallel to the long axis of the case, some is at right angles to the long axis and all vectors in between. If the pressure is adequate to deform the brass radially away from the flash hole (that is, pressure is making the primer pocket bigger), then why is it not sufficient to compress the base along the long axis of the case? What prevents that? I ask because if the brass deformed plastically in both dimensions then the primer pocket could get smaller.

In the end, I am curious but I don't really care. If I shoot loads that are too hot, my brass won't last. That is the long and the short of it.

 

[straightshooter1]

I don't understand the dynamics here. I accept that the barrel/chamber gets larger from pressure and that the brass is transmitting the pressure from the inside of the case to the steel. The powder makes 50k PSI, forces the brass against the chamber walls, the steel deforms elastically and the brass is along for the ride. The deformation exceeds the brass yield strength and the brass deforms plastically. Got it.

So I accept that the brass experiences plastic deformation - this explains why we have to resize our brass. I accept that full-length sizing reduces the size of the body - another (opposite) plastic deformation. I do not accept that full length sizing reduces the diameter of the case head. I have too many pieces of 300 win mag where the case had stopped fitting into the shell holder.

I can't figure out why the primer pockets get bigger. Why don't they get smaller? If the pressure is pushing on the the brass in the base, the base could get thinner and wider. That is, the base of the case would fit the chamber/bolt face more tightly because the diameter is larger but the distance from the bolt face to the top of the flash hole is less. If the brass is compressed, why did the primer pocket get bigger? Where did that brass go?

Maybe the case head just gets wider? Why would it compress in only one dimension? Some pressure is pushing toward the bolt face parallel to the long axis of the case, some is at right angles to the long axis and all vectors in between. If the pressure is adequate to deform the brass radially away from the flash hole (that is, pressure is making the primer pocket bigger), then why is it not sufficient to compress the base along the long axis of the case? What prevents that? I ask because if the brass deformed plastically in both dimensions then the primer pocket could get smaller.

In the end, I am curious but I don't really care. If I shoot loads that are too hot, my brass won't last. That is the long and the short of it.

Let's see if I can help . . . take a look at these pictures:

Notice the differences in thickness for the different area's on these various cases and the little differences in design. On the chambered case, note how much of a case in not inside the chamber.

Imagine the force vectors being applied to the surfaces from inside the various case designs. That unsupported area at the base of the case will expand due to pressure, thereby opening up the primer pockets. But how much and how fast it does that will depend on the thicknesses of the cases as well as to the particular hardness of the cases. . . and how hot the powder charges are that's being used.

My 2x fired .308 Lapua cases measure .469 at the case rim and 6x fired Lapua cases measure the same as does my 10x fired Federal cases. At the .020 mark of the web area, the 2x Lapua sized brass measures .4695, but the 6x fired Lapua brass measures .471 as does my 10x fired Federal brass. Primer pockets are all looser than they were when unfired, and the Federal brass pockets are more so, but not really bad a all (though my Federal brass hasn't been FL sized, only neck sized and shoulder bumped for the last 9 firings).

One of the things I notice when I firsts began uniforming my primer pockets after they've been fired was the floor of the primer pocket are pushed out a little, leaving a little bit of shavings each time I used the uniformer to clean the pockets. Pressure definitely effects that area of a case.

 

[Feniks Technologies]

I don't understand the dynamics here. I accept that the barrel/chamber gets larger from pressure and that the brass is transmitting the pressure from the inside of the case to the steel. The powder makes 50k PSI, forces the brass against the chamber walls, the steel deforms elastically and the brass is along for the ride. The deformation exceeds the brass yield strength and the brass deforms plastically. Got it.

So I accept that the brass experiences plastic deformation - this explains why we have to resize our brass. I accept that full-length sizing reduces the size of the body - another (opposite) plastic deformation. I do not accept that full length sizing reduces the diameter of the case head. I have too many pieces of 300 win mag where the case had stopped fitting into the shell holder.

I can't figure out why the primer pockets get bigger. Why don't they get smaller? If the pressure is pushing on the the brass in the base, the base could get thinner and wider. That is, the base of the case would fit the chamber/bolt face more tightly because the diameter is larger but the distance from the bolt face to the top of the flash hole is less. If the brass is compressed, why did the primer pocket get bigger? Where did that brass go?

Maybe the case head just gets wider? Why would it compress in only one dimension? Some pressure is pushing toward the bolt face parallel to the long axis of the case, some is at right angles to the long axis and all vectors in between. If the pressure is adequate to deform the brass radially away from the flash hole (that is, pressure is making the primer pocket bigger), then why is it not sufficient to compress the base along the long axis of the case? What prevents that? I ask because if the brass deformed plastically in both dimensions then the primer pocket could get smaller.

In the end, I am curious but I don't really care. If I shoot loads that are too hot, my brass won't last. That is the long and the short of it.

FWIW, there’s several reasons your 300wm brass eventually doesn’t fit in the shell holder.

And only one of those reasons would be that your FL isn’t sizing the entire body.

The rest are things like reaching its yield point, mismatched chamber to dies, chamber expansion, etc.


Fairly easy to check your die. Make a cast inside your die like you would a chamber. Measure it and compare to your chamber cast/reamer print and fired brass.

 

[alamo5000]

I am no scientist but this is my theory on the science of it.

A rifle case is essentially a brass tube with a head on the end of it. Firing a gun is a chemical reaction that causes a really violent reaction. Part of the chain reaction is when under pressure the brass expands out in every direction. It expands length wise as well as OUTWARD diameter wise.

To think this happens to every part of the case except the head is far fetched. While it doesn't happen immediately on the first firing (usually) the constant outward expansion essentially tugs on the case head. The case head is literally the only part of the brass that isn't resized either.

There are other things like brass moving around and 'flowing' through it's lifetime but by and large it doesn't take much 'stretch' to make it so that a primer will no longer friction fit into a case.

 

[918v]

I can't figure out why the primer pockets get bigger. Why don't they get smaller? If the pressure is pushing on the the brass in the base, the base could get thinner and wider. That is, the base of the case would fit the chamber/bolt face more tightly because the diameter is larger but the distance from the bolt face to the top of the flash hole is less. If the brass is compressed, why did the primer pocket get bigger? Where did that brass go?

If you want the sizing die to reduce the diameter of the case head you would need the die to be so much smaller than the case head that it would overcome the springback from the thick brass. But sizing dies don’t do that. They size the case enough to chamber and not enough to reduce the diameter below factory virgin specs. That’s why you see that bulge near the bottom of the case after firing and after sizing. And because there is always a difference in case head diameter before and after firing, the primer pocket keeps expanding.

With 300 WM brass you have a sizing problem because the sizing die has a recess at the bottom to accommodate the belt. And the case head design is such that the case walls at the case head are thinner than they would be if the belt wasn’t there. I have a RCBS small base die but it can only take the case down to .511” after it comes out if my .516” chamber. And that’s after sizing down in multiple steps using a roll sizer in conjunction with the sizing die. Now, with a .513” chamber the die works a lot better but still leaves the case bigger than factory.

So if you have a match between your chamber, the case, and the die you’re golden. If you have a mismatch then you’ll have problems.

 

[B y r o n]

Larry Willis collet die will help keep your belted brass chambering.

 

[Ledzep]

Your rounds are to hot is not the answer either. Thanks for any insight.

Wrong.

 

[Ledzep]

The internal pressure is the big part of this, as is the geometry of the inside of the case, the dimensions and condition of your chamber/bore, etc. Take a look through the pictures already posted in this thread-- especially those showing the exposed case sticking out of the breech.

The case that is inside the chamber expands, matches the chamber, then the chamber expands (very little, but it does). If you do a thick wall pressure vessel equation for state of stress and elastic deformation of the barrel (this is not exactly apples to apples because the firing event is very dynamic and the equations are for static loading), it indicates somewhere in the ballpark of .001" diameter increase on the ID of the bore. As pressure drops that bulge/expansion does so also until at the muzzle, the equations say it'd only be a .0001" or two. As has already been discussed, when the pressure drops, there is spring back and the brass usually pops free without issue, unless you expand the system so far that it creates an interference fit.

The portion of the case that is sticking outside of the breech is NOT supported by much. Portions of the bolt face, if anything. The case relies almost entirely on itself for support. This is why the 80ksi Sig NGSW have a stainless steel heads because the stainless has a yield strength that is multiple times that of the C260 brass used in cartridge cases. Unsupported, much over 60-65ksi causes plastic deformation in the head of the case. The loose primer pockets correlate to extractor groove expansion that can be measured with a blade mic. Some mfg's have more specific anneal cycles between draws/forming operations on the cases which allow for a higher state of cold work (hardness) in the head, which can handle higher pressures better. This balance of cold work and annealing is the "cost savings" mentioned above.

When loaded at 60-65ksi or less MOST lots of Federal, Remington, Hornady, Nosler, etc... (notoriously "soft junk" brass) will handle 10-20 firings. When searching for flat primers (68,000-75,000psi), ejector swipes (70,000-80,000psi) and other various indicators that guarantee you're well over SAAMI pressure, but many people seem to think are A Okay, it's easy to exceed pressures that the brass can handle for long life. Put simply, short primer pocket life is a sign of excessive chamber pressure or soft cases. In most instances, it's pressure that exceeds 65,000psi.

eta: Note on state of stress. Chamber pressure is NOT the same as the state of stress in the material. The geometry of the case will dictate much of this, but suffice it to say that if you have a material with a 70,000psi yield strength, it is not out of reason for some portion of the material to experience >70,000psi and plastically deform with a 65,000psi chamber pressure.

Also, not every shot is EXACTLY the same. If the average peak pressure for a batch of hand loads or factory ammo is 65,000 PSI, then you can expect a few of them to be 58,000 PSI, and likewise a few to be as much as 70,000 PSI. Just depends on the cartridge, powder, care taken in charging the case, etc... Peak pressure is also driven up and down by ambient temperature. This is why many will not grow, and a couple out of a batch will.

 

[tomme boy]

The brass when sized gets longer as that is the path of least resistance. The brass is pushed up into the body. And that is why they need trimmed. Just firing brass can actually make them shorter. And most brass is smaller than what the specs call for in diameter. That is also why you get the ring in the web area because the brass is made wrong. Win are notorious for this. And thats why brass like Lapua and Norma are so good. They are actually made like they are supposed to be.

 

[Simonsza1]

Wrong.

What I said is not wrong, I was asking for answers with some substance behind them and not a lame response like your rounds are to hot which is clearly what you probably would have written. I was asking what is actually happening to the pocket. That’s why I asked what’s the science behind it. Some folks here were good enough to try and explain with some real thought behind what takes place and not write a stupid remark. We all know hot rounds destroy brass in many aspects but hey thanks for your intelligent response.

 

[918v]

What I said is not wrong, I was asking for answers with some substance behind them and not a lame response like your rounds are to hot which is clearly what you probably would have written. I was asking what is actually happening to the pocket. That’s why I asked what’s the science behind it. Some folks here were good enough to try and explain with some real thought behind what takes place and not write a stupid remark. We all know hot rounds destroy brass in many aspects but hey thanks for your intelligent response.

Lol

 

[Haney]

Pressure pushes to the walls of a cylinder, like sealing the chamber when the powder is ignited . You can resize the soft brass of the body but not so much the primer pocket . Depending how hard it started out, thickness, and quality of brass will be a big factor in how long the pockets stay tight . Just another theory to ponder .

 

[GreenGO Juan]

My 2x fired .308 Lapua cases measure .469 at the case rim and 6x fired Lapua cases measure the same as does my 10x fired Federal cases. At the .020 mark of the web area, the 2x Lapua sized brass measures .4695, but the 6x fired Lapua brass measures .471 as does my 10x fired Federal brass. Primer pockets are all looser than they were when unfired, and the Federal brass pockets are more so, but not really bad a all (though my Federal brass hasn't been FL sized, only neck sized and shoulder bumped for the last 9 firings).

I find this to be the best way to determine if you have a hot load. If you are waiting till you blow primers out, you are already way over pressure.

I also find a correlation to this measurement and the tightness of primer pockets.

 

[bax]

Let's see if I can help . . . take a look at these pictures:

View attachment 7901522 View attachment 7901523

Notice the differences in thickness for the different area's on these various cases and the little differences in design. On the chambered case, note how much of a case in not inside the chamber.

Imagine the force vectors being applied to the surfaces from inside the various case designs. That unsupported area at the base of the case will expand due to pressure, thereby opening up the primer pockets. But how much and how fast it does that will depend on the thicknesses of the cases as well as to the particular hardness of the cases. . . and how hot the powder charges are that's being used.

My 2x fired .308 Lapua cases measure .469 at the case rim and 6x fired Lapua cases measure the same as does my 10x fired Federal cases. At the .020 mark of the web area, the 2x Lapua sized brass measures .4695, but the 6x fired Lapua brass measures .471 as does my 10x fired Federal brass. Primer pockets are all looser than they were when unfired, and the Federal brass pockets are more so, but not really bad a all (though my Federal brass hasn't been FL sized, only neck sized and shoulder bumped for the last 9 firings).

One of the things I notice when I firsts began uniforming my primer pockets after they've been fired was the floor of the primer pocket are pushed out a little, leaving a little bit of shavings each time I used the uniformer to clean the pockets. Pressure definitely effects that area of a case.

Thanks for the images. I hijacked one of them and I will use it to explain my question.

okay, the firing pin just ignited the primer, the powder is now in the form of a gas, it is very hot so the pressure is high. The gas atoms are pushing against the walls of the brass. For engineering purposes we can assume that the pressure vector is perpendicular to the wall - that is, straight down like the long red arrows, straight against the side walls for the orange arrows. The bolt face can be represented as a pressure vector opposite to the long red arrows. Hold off on the short red arrows.

Imagine a strange sort of sandwich - a piece of meat with a piece of bread on top. Spread mustard on the meat. Press down on the bread. The mustard will squirt out the sides - mustard yield strength is exceeded and it will flow from high pressure to low pressure.

Now think about the short red arrows. If the brass yield strength is exceeded the brass, like the mustard, will flow toward the weak areas not supported by the bolt face or the chamber walls. On the other hand, if the brass is strong enough, if its yield strength is high enough, it will temporarily elastically deform then return to shape. And that is mostly what we see. Brass can be reused several times without resizing the case head because the case head does not plastically deform - much. But not forever. Based on experience, some of the brass will exceed yield strength and will flow. In some cases the case head gets larger and since sizing does not apply to the entire case, after some firings the brass won't fit into the chamber or the bolt head anymore. So far, so good. To my mind, so far the theory and observations match.

[By the way, don't anneal case heads. It lowers their yield strength and one shot will wreck them.]

Now to the short red arrows representing pressure and force vectors in the brass. I am trying to say that the gas pressure increases the pressure on and within the brass and the brass deforms in the directions where it is not backed up by steel. But notice, right in the middle of the base is this hole where the primer goes - the primer pocket. Will the increased pressure cause the brass to deform into that hole? If yes, then that hole will get smaller and the primer pocket will get tighter. Is the primer itself strong? If yes then it resists the pressure and the brass deforms in other directions.

Back to our strange sandwich only this time no bread, it is made with a bagel. When you press on the bagel, the butter (it used to be mustard but who eats mustard on a bagel?) flows both outward and inward into the hole in the center.

And there is my question - why doesn't the primer pocket get smaller? I think because the primer resists. But I think that is not a completely satisfactory answer. Primers are not that "tough". Could it be that gas flows into the primer pocket and resists the deformation?

Mostly it doesn't matter but I wonder if case heads could be engineered so that primer pockets would not expand.

 

[GreenGO Juan]

 

[McCrazy]

View attachment 7902354

And there is my question - why doesn't the primer pocket get smaller? I think because the primer resists. But I think that is not a completely satisfactory answer. Primers are not that "tough". Could it be that gas flows into the primer pocket and resists the deformation?

There is an open flash hole going from the primer pocket into the case so the pressure pushing outwards in the primer pocket will be the same PSI as that in the case.

The primer isn't strong enough to contain much pressure so it mostly falls on the case head and bolt face.

Cratering and ruptured primers are obvious signs of primers exceeding their pressure limits.

But yes, the primer pocket absolutely holds pressure during the firing sequence which is the primary cause of primer pockets loosening.

Go ahead and add your arrows within the primer pocket.

 

[straightshooter1]

Thanks for the images. I hijacked one of them and I will use it to explain my question.

View attachment 7902354
okay, the firing pin just ignited the primer, the powder is now in the form of a gas, it is very hot so the pressure is high. The gas atoms are pushing against the walls of the brass. For engineering purposes we can assume that the pressure vector is perpendicular to the wall - that is, straight down like the long red arrows, straight against the side walls for the orange arrows. The bolt face can be represented as a pressure vector opposite to the long red arrows. Hold off on the short red arrows.

Imagine a strange sort of sandwich - a piece of meat with a piece of bread on top. Spread mustard on the meat. Press down on the bread. The mustard will squirt out the sides - mustard yield strength is exceeded and it will flow from high pressure to low pressure.

Now think about the short red arrows. If the brass yield strength is exceeded the brass, like the mustard, will flow toward the weak areas not supported by the bolt face or the chamber walls. On the other hand, if the brass is strong enough, if its yield strength is high enough, it will temporarily elastically deform then return to shape. And that is mostly what we see. Brass can be reused several times without resizing the case head because the case head does not plastically deform - much. But not forever. Based on experience, some of the brass will exceed yield strength and will flow. In some cases the case head gets larger and since sizing does not apply to the entire case, after some firings the brass won't fit into the chamber or the bolt head anymore. So far, so good. To my mind, so far the theory and observations match.

[By the way, don't anneal case heads. It lowers their yield strength and one shot will wreck them.]

Now to the short red arrows representing pressure and force vectors in the brass. I am trying to say that the gas pressure increases the pressure on and within the brass and the brass deforms in the directions where it is not backed up by steel. But notice, right in the middle of the base is this hole where the primer goes - the primer pocket. Will the increased pressure cause the brass to deform into that hole? If yes, then that hole will get smaller and the primer pocket will get tighter. Is the primer itself strong? If yes then it resists the pressure and the brass deforms in other directions.

Back to our strange sandwich only this time no bread, it is made with a bagel. When you press on the bagel, the butter (it used to be mustard but who eats mustard on a bagel?) flows both outward and inward into the hole in the center.

And there is my question - why doesn't the primer pocket get smaller? I think because the primer resists. But I think that is not a completely satisfactory answer. Primers are not that "tough". Could it be that gas flows into the primer pocket and resists the deformation?

Mostly it doesn't matter but I wonder if case heads could be engineered so that primer pockets would not expand.

As pointed out by McCrazy, that pressure moves through the flash hole into the primer area and pushes out as well. So the force pushing out of the powder chamber into the unsupported area along with the pressured in the primer area pushing out results in the eventual expansion of the primer pocket. And as I see it, the vector for that expansion is very much like you've illustrated in the two short red lines at the bottom of you pic.

Interesting, isn't it, that a non-expanding primer pocket has indeed been engineered?

 

[MarkyMark007]

And there is my question - why doesn't the primer pocket get smaller?

I think because it expand to the limits of the brass and a little more. Just like when you heat a metal with a hole in it; the hole gets larger and not smaller.

 

[Yondering]

I think because it expand to the limits of the brass and a little more. Just like when you heat a metal with a hole in it; the hole gets larger and not smaller.

No. It’s because there is pressure in the primer pocket pushing it outwards. With enough pressure, the pocket gets wider.

This is a very simple concept that a lot of you are trying to make more complex than it is.

 

[Ledzep]

What I said is not wrong, I was asking for answers with some substance behind them and not a lame response like your rounds are to hot which is clearly what you probably would have written. I was asking what is actually happening to the pocket. That’s why I asked what’s the science behind it. Some folks here were good enough to try and explain with some real thought behind what takes place and not write a stupid remark. We all know hot rounds destroy brass in many aspects but hey thanks for your intelligent response.

No, what you said was objectively wrong. If you would like a more in-depth reply then just ask for it. Don't be an askhole, asking your question by denying the simple answer. I regret taking the time to write post #65.

 

[Simonsza1]

No, what you said was objectively wrong. If you would like a more in-depth reply then just ask for it. Don't be an askhole, asking your question by denying the simple answer. I regret taking the time to write post #65.

Why? I thanked you for your intelligent response, #65

 

[Rockhurr]

My logic is the brass and primer, once fired, pushes back against the bolt face. Due to the force of the combustion, the primer flattens against the bolt face at the same time expands out pushing the primer pocket walls. Over time or with hotter loads the pocket enlarges resulting a loose pocket.

 

[Rockhurr]

My question to loose primer pocket is, how loose is too loose and can you still use them brass with loose or looser pockets as long as the primer doesn’t fall out upon shaking it. ? I have few 2nd firing Lapua brass that’s a lot looser than the rest but still fits snug enough and you can’t push a primer in without a primer seater… buts it’s looser than the rest… thanks in advance.

 

[Rockhurr]

Also, does loose primer pocket cause adverse effect on your shots, like a flyer, or potential gas blow back?

 

[Fig]

It's debatable. Litz says it doesn't matter. If it does it seems to be incredibly minor, like BR minor. The primer needs to stay in the pocket. It doesn't have to be "tight", but it shouldn't rattle, be loose, and nowhere near falling out. Lots of hand wringing and rumination has gone into primer seating, depth, and tightness. I'd say the majority of my brass ends up thrown away from loose primer pockets.

 

[hlee]

I’ve read that some will glue primers into loose pockets to get an additional firing or 2. But, that would really seem to mess with the ‘flow’ of reloading, and I’d have to ask how much my time is really worth. I think that a piece of brass with a pocket loose enough to require glue has done it’s job and can be retired to the scrap brass bucket.

The only other thing that I will add is that loose primer pockets increase the chance that the primer falls out upon firing (ejection). One of those little buggers down in your trigger can flat-line your rifle.

 

[Tokay444]

What exactly is happening that causes primer pockets to get loose? Also why so inconsistent? Seems like such a small amount of mine suffer. But mainly just looking for a detailed explanation of what is happening that causes this. Your rounds are to hot is not the answer either. Thanks for any insight.

The science is rheology.

 

[Tokay444]

Not asking about certain brass. Asking what it is in firing that is actually causing this event to happen and how it happens.

That would be pressure.

 

[RTH1800]

I did not read all the replies. Surprised so much discussion about a simple subject/question.

Primer pockets get loose due to case head expansion due to pressures beyond the elastic limit of the brass.

No more, no less.

 

[Tokay444]

"but what causes it?"

 

[Simonsza1]

The science is rheology.

Good word, bravo

 

[Near miss]

I was just coming here to post a question about this.

I have a lot of lapua cases, 5 times shot.

I marked the 4 cases that had most loose primer pockets of a lot of 28. I then reloaded and shot them along others.

Upon decapping there was no more difference between those. So the loose primer pocket might be because upon cleaning (getting sand out) some soapy water or just water entered the pocket. Or some other variable, even before seating the primer or the primer itself.

 

[straightshooter1]

My question to loose primer pocket is, how loose is too loose and can you still use them brass with loose or looser pockets as long as the primer doesn’t fall out upon shaking it. ? I have few 2nd firing Lapua brass that’s a lot looser than the rest but still fits snug enough and you can’t push a primer in without a primer seater… buts it’s looser than the rest… thanks in advance.

I've experimented a little about this and in my limited experience, as long as the primer isn't so loose (like ready to fall out), they work just fine. And as long as gas isn't blowing by the primer to the bolt face, it's all good.

 

[357Max]

The tale of 2 little shell piggies.

The first piggy made poor life choices. When the big bad powder wolf came it blew his primer out never to be seated again. It didn’t even have to huff and puff to do so.

The second little piggy grew up making good life choices. When the Powder Wolf came for him he laughed & withstood pressures that would make @Ledzep ass pucker!

What were these life choices. Well several are obvious from the pictures below. Some are not obvious (chamber & powder choice) yet interrelated & best left aside.

I thought it worth throwing out a few thoughts I haven’t seen mentioned.

Pressure is the obvious answer, but I’m also thinking about time & area on which the pressure acts.

The 6.8 SPC pictured is kind of a worse case for pocket life expectancy vs. the 6.5C Lapua that is optimized.

The circumferential wall surface area of the large primer pocket in the SPC is + 22% vs the small pocket in the Lapua. This is the radial wall of the pocket. The area the pressure acts on to stretch the pocket.

Pressure x area equals force. So given the same pressure say 58k a large primer pocket has to deal with 20% more force acting to permanently deform it.

The other obvious stand out in the picture is the huge flash hole on the SPC. .082 vs .063 on the Lapua. That’s +41% flow area.

So this early example SPC has a large pocket on which the pressure acts, reduced cross section due to the large pocket, & a huge flash hole further reducing cross section & allows the pressure to act more quickly. Bad juju!

Lapua has figured out that .063 doesn’t impede flash discharge at all. I’ve proven it myself testing the same load with GM205 vs CCI #41. The 41’s are the hottest small primers based on my unscientific testing via firing a variety of sml primers in empty cases. Gained 50fps with that primer switch all else same & that tells me the small flash hole is no impediment.

It can’t cost more money for the big manufacturer’s to follow Lapua’s lead & shrink the flash hole. Why don’t they?

Is there really any reason to run a large primer in any short action cartridge?

 

[hlee]

The tale of 2 little shell piggies.

The first piggy made poor life choices. When the big bad powder wolf came it blew his primer out never to be seated again. It didn’t even have to huff and puff to do so.

The second little piggy grew up making good life choices. When the Powder Wolf came for him he laughed & withstood pressures that would make @Ledzep ass pucker!

What were these life choices. Well several are obvious from the pictures below. Some are not obvious (chamber & powder choice) yet interrelated & best left aside.
View attachment 8118086View attachment 8118087

I thought it worth throwing out a few thoughts I haven’t seen mentioned.

Pressure is the obvious answer, but I’m also thinking about time & area on which the pressure acts.

The 6.8 SPC pictured is kind of a worse case for pocket life expectancy vs. the 6.5C Lapua that is optimized.

The circumferential wall surface area of the large primer pocket in the SPC is + 22% vs the small pocket in the Lapua. This is the radial wall of the pocket. The area the pressure acts on to stretch the pocket.
That difference is roughly the equivalent to necking a 223 up to 25cal. The effects of extra surface area on a bullet is pretty well known & same applies in the primer pocket.

The other obvious stand out in the picture is the huge flash hole on the SPC. .082 vs .063 on the Lapua. That’s +41% flow area.

So this early example SPC has a large pocket on which the pressure acts, reduced cross section due to the large pocket, & a huge flash hole further reducing cross section & allows the pressure to act more quickly. Bad juju!

Lapua has figured out that .063 doesn’t impede flash discharge at all. I’ve proven it myself testing the same load with GM205 vs CCI #41. The 41’s are the hottest small primers based on my unscientific testing via firing a variety of sml primers in empty cases. Gained 50fps with that primer switch all else same & that tells me the small flash hole is no impediment.

It can’t cost more money for the big manufacturer’s to follow Lapua’s lead & shrink the flash hole. Why don’t they?

Is there really any reason to run a large primer in any short action cartridge?

It might not cost more when setting up a line, but I’d imagine renovations to a line could be prohibitively expensive. Institutional inertia is the main reason I can think of for staying with an old way…

 

[Rockhurr]

I've experimented a little about this and in my limited experience, as long as the primer is loose (like ready to fall out), they work just fine. And as long as gas isn't blowing by the prime to the bolt face, it's all good.

Thank you! Threw out 5 last year… had 5 here ready to go too but you saved them. Lol.

 

[RTH1800]

Federal has always been soft. What's your buddy's source that other companies are making their brass softer? Did they tell him? Does he test the brass for hardness?

Just being a ballistician doesn't mean that he has access to what is almost certainly proprietary manufacturing knowledge from companies he doesn't work for.

 

[tomme boy]

If you know someone with a roll sizer it can make the primer hole tighten back up for a few more loads.

 

[918v]

I have a roll sizer and no it cannot.