?

Hey Alf

My one and only case head separation looks amazingly similar



It has an identical indentation. I am at a loss to explain it. This was on the 6th reloading of a WW 338 win mag case. I was loading over book max the same as you.

Of course we all have heard of the explanation of case head separations in that it is thinning above the pressure ring caused by the initial firing. The firing pin slams the case forward to contact at the shoulder, the case expands to the chamber walls and grips and then the pressure forces the case head back to the bolt. The case thins above the pressure ring.

How much excess headspace do you have on new cases? On this particular gun new cases expand from 2.097" to 2.120" as measured by a Head & Shoulder Gauge. That is .023" which is almost twice as much as on an unbelted case like a 30-06. That initial expansion and full length sizing every firing will hasten case head separations.

Do you headspace on the shoulder?

?

Don't know if you know this so don't take offense please.. if you do.
Seen this story numerous times. If you fire a belted mag then resize it fully all the way down till the die stops on the shell holder this will happen more often then not. The trick is after the initial firing to only partial resize. I do this by starting with the die raised slightly above the shell holder then making sure I have case lube on the neck and body (not the shoulder) I run it thru. If the lube is still a ways above the shoulder on the neck then turn it down a bit more until it just meets the neck/shoulder junction. Once you have it just sizing the neck without pushing the shoulder back its set right. Basically now it headspaces on the shoulder and the case stretch stops. Now of course clean the case and function test it. Should be fine.
Where I guess things go wrong is that most die instructions tell you you must have the bottom of the die contact the shell holder. That you should feel a little compression from it before you put a shell thru the die to resize it. This only works for some chambers (short ones) most long mags don't like this system in my experience.
Theres 3 ways to size brass as I understand it
1)- Full resize (tight chambers, Autos, lever guns)
2)- Partial resize (what I just tried to explain)
3)- Neck sizing (target style)

Most belted mags use the same headspace spec think it is .220"min .227"max and that is measure from bolt face up if the gunsmith reams the chamber longer you just increase the .227" and that would increase OAL on the case. You can measure a fired case from rim to shoulders and compare that against an unfired case and adjust the die. Another problenm is using an expander ball as you pull the expander out it can move the neck/shoulder forward your die acts like an accordion so you really need to lube the inside of the necks.

I had Custom Product build me this die
http://www.neiljones.com/html/micro_dies.html works great for the belted mag.

I had a mag build last year and I had them headspace on the shoulders gets rid of the problem with the belt.

Well good luck

Common enough problem. Best solution is to neck size which will require a neck size die. Then when cases need more sizing because the bolt is hard to open and close just partial resize. To partial resize adjust the FL die out so the case shoulder is set back enough to just fell the bolt close on the case when chambered. That way the case is headspacing on the shoulder instead of the belt.

If you don't want to neck size then just partial resize as mentioned. You will still get head seperation after a while but just not as soon.

Larry Gibson

Still want some thoughts on the dent myself, that I can't figure--is that somehow related to case stretching, which is defined as going on?????

Alf, my 308 did the same thing (without the dent) with 44gr of varget and a long seated 175. We closed and lock the bolt on a no-go gauge. I got the gun used and had put about 600 rounds down it when it came apart. Now it has a Shilen on it.

My vote is on excessive head space in the gun.

Just some thoughts here as to the "dents"- work hardened case necks, delayed ignition-switch to a mag primer. The dents are usually accompanied with some soot, which I don't see present in the photo, but are caused by the neck not expanding fast enough to seal, personally I like a mag primer with 7828 in even smaller cases like 06 Ack. Another thing I would check on the rifle is a long or erroded throat .02

Okay guys, I'm way ahead of you here

quote:

Another problenm is using an expander ball as you pull the expander out it can move the neck/shoulder forward your die acts like an accordion so you really need to lube the inside of the necks.

Use a Lee Collet Neck Sizer for all my reloading so I don't need lube in the neck and I have no expander ball pulling on the neck.

quote:

Best solution is to neck size which will require a neck size die. Then when cases need more sizing because the bolt is hard to open and close just partial resize. To partial resize adjust the FL die out so the case shoulder is set back enough to just fell the bolt close on the case when chambered. That way the case is headspacing on the shoulder instead of the belt.

As mentioned I use a Lee Collet and monitor my shoulder position so that when it gets tight I set the shoulder back .00015" for a very slight crush fit with a Redding Body Die so I am headspacing on the shoulder as soon as possible.

quote:

My vote is on excessive head space in the gun.

After the first firing excess headspace essentially disappears but I have had a gun (pre 64 Model 70 264 win mag) with excessive headspace and it is more likely to do this



burn through at the shoulder. Excessive headspace does lead to more thinning of the case at the place where these case head separations occurred though, I agree with that. But it is supposed to then be mitigated by neck sizing and headspacing on the shoulders.

quote:

delayed ignition-switch to a mag primer

I used Fed 215 M primers in all my belted mags.

quote:

Still want some thoughts on the dent myself, that I can't figure--is that somehow related to case stretching, which is defined as going on?????

That's the question. If a cause could be assigned to the dent that may lead to a cause of the case head separation. Until Alf came along with his photos I thought the dent was an anomaly and possibly caused during ejection.

If the neck is not expanding fast enough to seal the chamber, why would it put a dent like that there and without a lot of soot?

About the dents. Have you tried tech support at Sierra, RCBS or another big player?

Just a thought.

Brian

It's just as Westcoaster described. Since belted magnums headspace on the belt the chamber tends to be cut overly large in the shoulder area. Upon first firing the case fills that extra space and when you full length resize the case it pushes the shoulder back into spec. Do this three or four times and the case seperates where it's been worked, right where you see. My gunsmith got tired of pulling stuck cases out of the chamber of my 7mm rem mag before I figured this out. The solution is partial full length resizing as mentioned. The best way to do it is to buy a set of stoney point headspace gauges and set your dies to bump the shoulder back .001-.002". It's very easy to see with the gauges and your brass will last forever doing it this way.

Hey Don and Woods, The "Case Wall Dent" is created by the Gas which escaped when the Head Separation occurred. The Bullet moved a bit forward, the unburned Powder slightly plugged the Neck/Bore and the Pressure got behind the Case which began crushing it in the thinnest spot.

I believe someone, long ago, once offered a "Reflected Pressure Wave" as an answer, but it has been so far in the past that the specific memory is possibly Full-of-Beans.
-----

P-FLR will correct it. But if the rifle is used for Dangerous Game, then the Cases need to be FLRed and tossed into the Practice Bin after 2 firings and then P-FLRed.

Nothing tricky, just Resized and Reused too much for that Chamber & Die combination.

quote:

Originally posted by Hot Core:
Hey Don and Woods, The "Case Wall Dent" is created by the Gas which escaped when the Head Separation occurred. The Bullet moved a bit forward, the unburned Powder slightly plugged the Neck/Bore and the Pressure got behind the Case which began crushing it in the thinnest spot.

I believe someone, long ago, once offered a "Reflected Pressure Wave" as an answer, but it has been so far in the past that the specific memory is possibly Full-of-Beans.
-----

P-FLR will correct it. But if the rifle is used for Dangerous Game, then the Cases need to be FLRed and tossed into the Practice Bin after 2 firings and then P-FLRed.

Nothing tricky, just Resized and Reused too much for that Chamber & Die combination.

My thoughts as well.
In my 7mm Rem Mag brass lasts a long time before I need to chuck it... using this method.

HC, that seems to make sense! Hadn't seen or heard of it before. I think maybe there is a lot of room in that particular shoulder....

Had the same problem with my 7mm STW. I was probably overworking the brass creating a thin spot right above the belt. The hard part is getting that case out of the chamber. I changed my die to slightly bump the shoulder and limit my loadings per case.

Ok guys, you're losing me now I think I need some help!!!

Woods is NS after the first firing and is monitoring when he needs to P-FLR to bump the shoulder back if I understand this correctly...

Hot Core is saying this could be remedied by simply P-FLR the case...OK so far?

But, doesn't the shoulder get the crush fit after a couple firings and then bumped as Woods is already doing?

For example NS, NS, then P-FLR to bump the shoulder?

How is one to P-FLR from the beginning if the shoulder hasn't expanded enough yet to touch the shoulder that's one of the areas I'm getting lost in...

Help please HC, Woods, and anyone else for I want to start reloading my 338WM and 7RM without blowing myself to smithereenz...

Sorry, but just a newbie newbie and I know this is some real advanced stuff but I want to learn...

Thanks to all...

Hey LB

You got it right. Don't listen to anyone else, especially HC. Just kidding!

It is possible to hasten the shoulder movement by using your full length die and when you size the case body it will push your shoulder forward some. I have never found a case that where you could move the shoulder all the way to a crush fit by sizing the case body on once fired cases though.

So I do fire, NS, NS and then PFLR like you said. It's the best you can do short of a hydraulic form die.

Alf, here's a question for you regards the dents...

Are the dents on every case? More importantly, do you experience the dent on the first case fired after the rifle and chamber have been cleaned?

If one is using a too slow burning powder which may leave an unburned powder granule or two in the bore, it is possible that a grain or two may get in the chamber between the front of the case and the chamber wall. If that happens, ignition of that granule(s) may very well dent the case.

I have a .30-BR in which I can produce such dents on demand by using too slow a powder and leaving unburned grains in the bore.

Doesn't seem like that would be likely with a hot load, but then, who knows? Anything else in there that is flammable and which produces gas when burned, might be able to do it too.

ALF

Had the same thing happen with Rem brass in .375 WeM. Worked out that it was due to the bullet being pushed back into the shell -- third or fourth in magazine stack. Recoil from previous shots shoved 350-grain Woodleighs into shell despite being crimped at groove. I have to choose full or compressed loads with IMR 4831 instead of the W760 ball powder used for the load.
Third reload, with Remington brass (JUNK!!!) and no problems since. Were you shooting singles or filling the magazine when shooting? Just my thoughts and experience...

BNagel

try running the brass over a next size larger diameter expander ball.Then resize with the proper die turned out a turn or so.notice the case will not chamber.keep turning the die in a quarter turn and retry,until the case is a tight fit in the chamber.now your shoulder is in the right place,and your brass will last much longer.I have a few die/rifle combos which if I screw the die down to the shell holder,will allow the primers to back out some on firing.(I dont shoot any belted cases)
Bob Hagel has some books which cover the subject well,as does Parker Ackley.

quote:

Originally posted by Late-Bloomer:
...Woods is NS after the first firing and is monitoring when he needs to P-FLR to bump the shoulder back if I understand this correctly...

And getting some excellent groups, especially considering the erratic Powders he is using.

quote:

Hot Core is saying this could be remedied by simply P-FLR the case...OK so far?

That is in response to "how" the large dent occurred on the separated case. I'll expand on it a bit more below.

quote:

But, doesn't the shoulder get the crush fit after a couple firings and then bumped as Woods is already doing?

For example NS, NS, then P-FLR to bump the shoulder?

I believe that is similar to the sequence Woods uses.

quote:

How is one to P-FLR from the beginning if the shoulder hasn't expanded enough yet to touch the shoulder that's one of the areas I'm getting lost in...

I can see where that is confusing. I just don't change the Die from the P-FLR position when I have new cases. You are correct that the Shoulder is not Pushed back(if the Factory made them correctly).

The thing you are missing as far as my Cases, is that I do some kind of "Fire Forming". Normally just Seat as cheap a Bullet as I can find so it is about 0.010" Into-the-Lands. And I use a Load strong enough to Expand the Case properly.

Or I occasionally Expand the Neck to a Larger size then P-FLR the Case and that creates an Expanded Neck behind the Bullet that causes the Crush Fit.

You can start with a Larger Case in the beginning and just P-FLR it down to the size you want, like converting a 308Win to a 7mm-08. But I normally just Fire Form with a cheap Bullet.

quote:

... I want to start reloading my 338WM and 7RM without blowing myself to smithereenz...

Lots of folks on here can help you. That is the great thing about this Board.

quote:

Sorry, but just a newbie newbie and I know this is some real advanced stuff but I want to learn...

No need to be sorry. I'd guess 100 or so years ago even AC was a Beginner.
-----

Time 0

I'll see if I can expand a bit on my thinking. ANYONE(including alf) that disagrees, feel free to jump in and correct me if this is Full-of-beans.

CH______PR______________________________________________________________________________________________S___N___M
__T0____________________________________________________________________________________________________________

CH=CaseHead, PR=Pressure Ring, S=Shoulder, N=Neck, M=Mouth, and T=Time in Microsec. D will =Dent, PB will=Progressive Burn, and CHS=CaseHead Separation.

The Bullet is not shown because the AR Software won't allow me to use Spaces. The above represents a Loaded Cartridge.
-----

Time 1

CH______PR_PB_____________________________________________________________________________________________S___N___M
___________T1_____________________________________________________________________________________________________

At T1 the Primer has Fired. The Powder is Progressively Burning Forward as Pressure begins to build.
-----

Time 2

CH______CHS______PB_______________________________________________________________________________________S___N___M
_________________T2_______________________________________________________________________________________________

At T2 The Powder is forced forward by the Pressure and Expands the Case enough to create the CHS. The Powder moving Forward holds the Shoulder tight against the Chamber Shoulder and the Bullet is in the beginning of the Barrel, beginning to accelerate.

However, since the CHS occurred, the Pressure can now move outside the Case. The PB has created somewhat of a moving Wall as the Powder continues to Burn. So the Pressure is going outside the Case and begins to fill the Void between the Case and Chamber.
-----

Time 3

CH______CHS___________________________PB_____________________________________________________________D____S___N___M
______________________________________T3__________________________________________________________________________

The Pressure has moved ahead of the PB and bears on the Case at its weakest point between the PB and S creating a Compression Dent(s). Pressure behind the PB has no effect on the Case Wall. That is because the Pressure is the same inside and outside the Case behind the PB.
-----

Time 4

CH______CHS__________________________________________________________________________________________D____S___N___M
________________________________________________________________________________________________T4________________

At T4, the Pressure is already decreasing because the Powder has been consumed. There is not enough Pressure remaining to "Snap Out" the Dent(s) behind the Shoulder.
-----

That is how I see it. Feel free to correct me. But please give a detailed explaination of your thinking.

Best of luck to you folks.

quote:

Hey Don and Woods, The "Case Wall Dent" is created by the Gas which escaped when the Head Separation occurred.

This sounds like a reasonable explaination for the dent.

One condition causes this. You are setting your cases' shoulders back too much when sizing them. (They may be too short to begin with, which results in the initial stretching. But cases don't usually fail on the first firing, even if prettty short in head-to chamber shoulder dimension. It takes several repetitions to reuslt in the whole head coming off. BUT, you can see when the case head is starting to separate-you get a nice, flat-looking shiny ring on the outside of the spot where th separation is occurru=ing inside. When you see this warning sign, you can usually feel the growning chasm inaside the case with a bent paperclip used to feel for the depresion opposite the shiny ring.)

This could be due to a too-short head-to shoulder length in your sizing die, or the head to shoulder length of your rifle's chamber is excessive. Case head separations are usually due to excessive lengthwise stretch of the brass, not due to too much radial growth. But, if asked to stretch too far to reach the chamber walls, the head could blow! You will sure know it if that happened, even before you opened the bolt!

To stop this, set your FL die to only size about one bullet diameter's amount of the case neck-just enough to firmly grasp the bullet.

I've had the same thing happen with the .348 Winchester brass in a Model 71. Excessive stretching was caused by the springy lockup of this action.

Dents-those look to me like cave-ins, and such things occur when pressures are TOO LOW!

Almost every belted case will not match the chamber to headspace on the belt. You MUST headspace on the shoulder after the case is fire formed. Never push the shoulder back and rely on the belt, it just doesn't work and you will stretch the brass ahead of the belt.
The other two cases in Alf's picture are already showing signs of failure.
Most of the time this is not a headspace problem but variations in belt width and chamber specs.
Remember, this belt was based on one shot from the brass with factory loads.
Any handloader knows to not set back shoulders any more then it takes to chamber a round without a lot of force. The belted case MUST be treated exactly the same. You should feel a little resistance when closing the bolt.
In other words, you need to switch to shoulder headspace instead of belt headspace.

quote:

Originally posted by El Deguello:

To stop this, set your FL die to only size about one bullet diameter's amount of the case neck-just enough to firmly grasp the bullet.

Hot Core or anyone else...

Wouldn't this work also for guys like me who aren't fire-forming their brass initially, i.e., after the first shot, adjust my FLR die to re-size only a bullet-diameter of the neck... that way the rest of the neck is tighter and the shoulder should be very close to head-spacing off the chamber's shoulder because of this expansion (bulge)...???

So in effect there will be much less stretching of the case and head case...

Or, am I still at a loss in following this happy trail?

Thanks for your patience and guidance as this STUFF "Head-Case Separation" gives me the willies as a newbie newbie...

Roland

quote:

Originally posted by Late-Bloomer:

Hot Core or anyone else...

Wouldn't this work also for guys like me who aren't fire-forming their brass initially, i.e., after the first shot, adjust my FLR die to re-size only a bullet-diameter of the neck... that way the rest of the neck is tighter and the shoulder should be very close to head-spacing off the chamber's shoulder because of this expansion (bulge)...???

So in effect there will be much less stretching of the case and head case...

Or, am I still at a loss in following this happy trail?

Thanks for your patience and guidance as this STUFF "Head-Case Separation" gives me the willies as a newbie newbie...

Roland

I find what you suggest works fine in my rifles.

in that instance, even though some areas of the die walls may contact some sections of the cartridge case walls, usually the lower parts, NONE of the die will contact the surface of the case shoulder. So, the die will not be able to move the shoulder back any amount at all.

Therefore the sized case(s) will be essentially headspacing on the shoulder. Not a crush fit, but not far from a perfect fit anyway.

You will, of course get a little stretch of the brass near the base on the very first firing, but you won't be continually setting the brass back then blowing it forward again. That constant working of he brass by set-back and blow-forward can be a real bummer regards brass life.

Make yourself a feeler wire. Cut off a section of a coat hanger or straighten a paper clip. Bend a tiny right angle hook on the end of it maybe only a 1/16" long. File a rounded edge.

Reach inside of every case to determine if it starting to separate. This method will detect pending failures long before they show up on the outside of a case.

Such inspections and subsequent correct FL die adjustments will result in longer case life and greater safety.

quote:

Originally posted by Late-Bloomer:

Wouldn't this work also for guys like me who aren't fire-forming their brass initially, i.e., after the first shot, adjust my FLR die to re-size only a bullet-diameter of the neck... that way the rest of the neck is tighter and the shoulder should be very close to head-spacing off the chamber's shoulder because of this expansion (bulge)...???

So in effect there will be much less stretching of the case and head case...


Roland

Hey LB, while leaving part of the neck fire formed size might be a good idea, it will not keep the firing pin from slamming the case forward if there is headspace there



Forster even makes a bushing / bump die to do exactly that. This is an excellent way to mitigate or alleviate the bad effects of a donut on bullet grip. You can do the same thing with a Lee Collet Neck Size Die and a washer around the case head on top of the shell holder



I don't know why I had the case head separation but I can tell you one reason that it is not; pushing the shoulder back too far when sizing.

quote:

I just don't change the Die from the P-FLR position when I have new cases. You are correct that the Shoulder is not Pushed back(if the Factory made them correctly).

Hey HC, if you had the appropriate "thingy" you could actually measure the shoulder position.

Hey Woods,

Is that the way you NS til you bump the shoulder back?

Thanks for the diagram and the tip about being able to use my Lee Collet die with the washer...

quote:

Originally posted by woods:
...I don't know why I had the case head separation but I can tell you one reason that it is not; pushing the shoulder back too far when sizing.

Seems like there is always someone finding a new way to break an anvil.

quote:

...I have had a gun (pre 64 Model 70 264 win mag) with excessive headspace and it is more likely to do this

Is this the particular rifle you are refering to? If it is, BH can probably tell you what the problem is.

Normally, there has to be some stretch at the Pressure Ring coming from somewhere. Did you Fire Form with a Bullet Jammed into the Lands when the Cases were new?

quote:

...Hey HC, if you had the appropriate "thingy" you could actually measure the shoulder position.

Kind of like measuring Velocity, once you have done it and have the number, you eventually realize it is pretty much worthless, time-consuming drivel.

Speaking of "breaking anvils", is that Tomato dirt on the fine stainless Lee Die, or have you also figured out a way to make them Rust?

quote:

I am at a loss to explain it. This was on the 6th reloading of a WW 338 win mag case. I was loading over book max the same as you.

Woods, after 6 loads how many times had you trimmed that brass, at least 3 times I'll bet, maybe more, the extra brass that you have to trim is coming from somewhere, now you know where- where the case seperated, it flows too.

quote:

Originally posted by Alberta Canuck:
........
it is possible that a grain or two may get in the chamber between the front of the case and the chamber wall. If that happens, ignition of that granule(s) may very well dent the case.........

That is the only explanation that accounts for the 'unbalanced' pressure that may dent the case inward, toward the pressure side of the case wall.

quote:

Originally posted by swheeler:

quote:

I am at a loss to explain it. This was on the 6th reloading of a WW 338 win mag case. I was loading over book max the same as you.

Woods, after 6 loads how many times had you trimmed that brass, at least 3 times I'll bet, maybe more, the extra brass that you have to trim is coming from somewhere, now you know where- where the case seperated, it flows too.

Maybe so, it is hard to keep track of the times a case is trimmed because typically I just stick a Lee Case Length Gauge trimmer in it and sometimes a little comes off of some cases. I do not recall any brass that had a lot of brass coming off the neck in a trimmer though.

Woods; now I will throw in something else to think about, pertaining to the dents. Let us say that Hot Core is correct in his explanation for the dent in the shoulder area, only this time ONE EXTRA event occurs, at the time of head seperation the pressure falls enough(remember the dent is caused by gas leaking from the seperation into chamber around the casing- chamber capacity is larger than internal brass capacity by some stretch and the bullet is already moving down the barrel creating an even larger combustion chamber)pressure drops enough to let the powder slow or almost stop burning, bullet stops in the barrel, powder starts burning again and resistance from barrel obstruction(stopped bullet)creates a "bomb", maybe that unexplainable SEE? MAYBE YOU ARE JUST EXTREMELY LUCKY! No reply needed, just rambling on

quote:

Originally posted by ALF:
Rifle Remington 700 BDL
Cal: 7 mm STW
Bullet: Hornady Interlock 162 gr
Case: RP 4th reload with full resize between shots
Primer: Fed: 210
Powder: IMR 7828 76.5 gr

Velocity range for 3 shots 3011 fps, 3009 fps, 3018 fps Av 3018 fps. SD 15

After firing the case was stuck, when it was punched out using a cleaning rod the head came out and the rest was removed with a shaped dowel.

It would appear from case marked 1 ( reloaded once and case marked as 3 , reloaded 3 times that the case is narrower just ahead of the belt and that the expanded area in contact with the chamber is wider at the head, the separation occurred 2 to 3 mm in front of this area.

Also look at the dents in the are just before the neck, there is a smaller one on the other side

Any comments?

I believe the crushing of the case is caused by gas pressure from the rupturing of the case.The gas comes back to the bolt and blows forward to escape crushing the case in the process. This is how it was explained to me by an old reloader,maybe he was right,it sounds like a reasonable explanation.

quote:

Originally posted by ALF:
Rifle Remington 700 BDL
Cal: 7 mm STW
Bullet: Hornady Interlock 162 gr
Case: RP 4th reload with full resize between shots
Primer: Fed: 210
Powder: IMR 7828 76.5 gr

Velocity range for 3 shots 3011 fps, 3009 fps, 3018 fps Av 3018 fps. SD 15

After firing the case was stuck, when it was punched out using a cleaning rod the head came out and the rest was removed with a shaped dowel.

It would appear from case marked 1 ( reloaded once and case marked as 3 , reloaded 3 times that the case is narrower just ahead of the belt and that the expanded area in contact with the chamber is wider at the head, the separation occurred 2 to 3 mm in front of this area.

Also look at the dents in the are just before the neck, there is a smaller one on the other side

Any comments?

I would think you would get more velocity than what you posted with the STW,I get 2930 fps with a 190 grain Hornady BTSP out of my 300 Win.....

The dent is caused by the separation, and not by an under-pressure situation.

When you get an under-pressure dent, as mentioned by several people you not only get a black soot, but the dent looks different than a separation dent like this.

In addition to a sooty neck, the dents look more like dents from excess lube (well to be accurate it would look like one from a LOT of excess lube) But the main thing is you can look at the dent and see how it originates at the front and moves to the rear. Look at this case, and it clearly starts at the back (notice how the dent is shallower) and then moves forward, even pushing past the shoulder.

What I suspect but don't know for certain is this dent is located directly forward of where the case initially failed. Woods' picture seems to support this but to be honest I don't think a pic would give enough detail to verify with certainty.

Now again, why it separated has already been discussed, and is a different issue.

Mark, are you saying that gas is trapped between the chamber and the case at the same pressure as inside the case, then as the pressure falls within the case (as at bullet exit and/or case rupture) the trapped gas expands, unbalanced, into the case wall, causing the dent?

303guy,

You ask a good question, and to be honest I don't know enough to give you a good answer!

One problem is the pressure, for lack of a better term is more "dynamic" than "static" like an expansive gas already under pressure, say like opening a bottle of soda. I suspect the case most likely does not fail at exact peak pressure, but some time before that point so it is still expanding. It may even be possible for the gas pressure outside the case to "outrun" the pressure forming inside the case and cause this, but again I'm not at all qualified to analyze how this occurs, hopefully we can get an answer from someone who does!