.

Hey Grips

You have been reading too much. I don't know Varmint Al, but it sound like BS to me. The case needs to grip the chamber wall to seal, when it does not you get a smoked up case on the outside surface. when the cartridge fires pressure is equal in all directions so if the cartridge produces 40,000 psi that is 40K at all points inside the chamber. If the chamber is smooth or rough makes no material difference. A rough chamber makes extraction more difficult. Case stretch is a product of pressure and bolt movement, not chamber surface.

Manufactures have a bunch of high paid engineers who decide how to build rifles, does Al think that all of them are wrong?

:hmmmm:

 

Well, Al uses Finite Element Analysis (a field in which he apparently has 20 years of experience) to model the barrel/chamber/casing/bolt interaction. He presents a strong argument, and provides all his research notes and computer-generated analysis that case-head separation is a factor of non-uniform case stretching. Because the neck area is considerably thinner than the base, separation of the base without appreciable thining of the rest of the case suggests that the case material was somehow restricted to flowing only at the base. In other words, all the PSI had no place to stretch the casing except at the base. This could only happen if the rest of the case were held firmly in place by "something", said "something" being the chamber wall.

Specifically, if the case walls do not stretch because-and-when they "stick" to the chamber walls, then ALL of the case expansion MUST be at the case head (why is the "head" also the "base"? You know, we "headstamp the base"?). Even though that is the thickest part of the casing, by stretching only there the case head will separate sooner (in reloading) than will a case that has enough "slip" on the chamber walls to allow proportionately-uniform case expansion.

You ought to see the FEI animation of the analysis. Al brings up a good point.

Note that field experience tells us that dirty chambers exacerbate the prevalence of case-head separations. Likewise, "Never polish your chambers or slick them with Moly" comes out of serious bolt/receiver damage using factory loads.

Al's focus was on the scenario where too little bolt pressure was exherted as a result of intentionally roughening the chamber wall - resulting in very rapid (as short as on the second reload) case-head separation. Al also demonstrated the alarming increase in bolt pressures by replicating stages of increasingly-smooth chambers.

My question sort of revolves around "Does anybody know what the factory specs are for 444 XLR bolt pressures?" I'm interested in knowing just how much fluff-n-buff I can apply to the chamber before I'm encouraging beyond-factory-design expansion pressure on the bolt alone.

So far, I've found this reference:
From PapaJohn's Re: Prepping New Brass
page 3 posting 04-16-2011, 09:19 AM
"Then resize and tumble for a few minutes to get the lube off. Some folks say it's not necessary, but I don't want slick shells in my chamber, increasing bolt thrust. In a Marlin, extra bolt thrust is NOT a good thing."


I'm not so sure about "reading too much." That's the process needed to unearth questions that otherwise would not occur to me. Granted, I later discard most of what I read, but otherwise I'd still think the world is only as big as I can see it.

Grips

 

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Hi Grips,

I agree with your facts, where I don't agree is with your/Al's conclusions drawn from the facts. There is no way that slicking up the chamber can direct pressure to the bolt face. That would be counter all the physics that I have known. Take for example the cartridge that produces 40K of chamber pressure. That pressure is in a contained space (the rifle chamber) and that 40K applies to all surfaces equally. Since every action has an equal and opposite reaction (Newton's law), that means that if we increase the pressure on the bolt by 10K then we also increase the pressure on the bullet and walls by 10K, so now we have 50K of pressure, not logical. Pressure inside an enclosed chamber can not be moved around to apply more at one point and less at another point.

The pressure of a 30-30 cartridge averages about 40K. the cartridge is .506 inches at the head (inside the case is somewhat less so we can call it .500 for example). So we have a half inch circle or about .2 inches of area or 20 % of 1 inch to which pressure is applied. 20 percent of our 40K is 8K or, our bolt "sees" about 8000 PSI, so then how much more does it receive if we polish the chamber walls and where does that added pressure come from? If 2000 additional PSI is added to the bolt face as a result of polishing the chamber then 2000 PSI would have to be taken from the base of the bullet and we would lose velocity.

This is no reflection on you, Grips, I have heard this myth repeated many times. I am sorry, I just cannot buy into it. If you do then we just have to agree to disagree.

:tee:

 

I have read several of "Varmint Al's" stories over the years. As JB said, physics is physics. Al had an article a while back about Nickle plated cases, and "prepping" them. That article was LOADED with bad assumptions based on no facts, just based on what he didn't like.

Smart people can be dumb to, just ask me.

 

Hey guys, I agree that the pressure stays the same. What Al suggests is that the vertical "flow" of the brass itself, responding to the pressure, will (if most of the brass is held so it can only expand out, but not up/down) cause the bottom of the case to move/flow/stretch *disproportionately*, and would express that movement as vertical case-stretch only at the case head/base.

Thus, separation could occur even though the nominal internal pressure limits were not exceeded.

This would be like when you cup your hands around a balloon and blow, you can keep the "big area" small, and get a bulge in the "small area". You have not blown any harder, but you have redirected the pressure to a spot the balloon engineer did not intend the pressure to go.

Hmmm, combining that balloon analogy with Al's evaluation, maybe the only serious problem is a sticky chamber. Could it be that a slick chamber would encourage case vertical stretching mostly at the thinest points? Stretch is definitely part of the pressure equation, and it would seem that an unsupported case would stretch most at the thinnest part (the neck).

This is not my first encounter with Al's works. I'm not trying to support them. However, I do think he's on to something of value. I think we can agree that sticky chambers are Not A Good Thing. I'm just turning the equation over to ask "Could it be that slick chambers could be bad too?"

Grips

 

Hey guys, I agree that the pressure stays the same. What Al suggests is that the vertical "flow" of the brass itself, responding to the pressure, will (if most of the brass is held so it can only expand out, but not up/down) cause the bottom of the case to move/flow/stretch *disproportionately*, and would express that movement as vertical case-stretch only at the case head/base.

Thus, separation could occur even though the nominal internal pressure limits were not exceeded.

Correct, so far.

This would be like when you cup your hands around a balloon and blow, you can keep the "big area" small, and get a bulge in the "small area". You have not blown any harder, but you have redirected the pressure to a spot the balloon engineer did not intend the pressure to go.

Hmmm, combining that balloon analogy with Al's evaluation, maybe the only serious problem is a sticky chamber. Could it be that a slick chamber would encourage case vertical stretching mostly at the thinest points? Stretch is definitely part of the pressure equation, and it would seem that an unsupported case would stretch most at the thinnest part (the neck).

This is not my first encounter with Al's works. I'm not trying to support them. However, I do think he's on to something of value. I think we can agree that sticky chambers are Not A Good Thing. I'm just turning the equation over to ask "Could it be that slick chambers could be bad too?"

Grips

Here is where we disagree.
The slick or rough chamber has nothing to do with case stretch. Case stretch is a function of pressure and movement of bolt face. As pressure builds in the case, the case is sealed to the chamber walls from case mouth to back end of chamber. The seal is not made from back or chamber (in the barrel) to the bolt face. As pressure continues to build the tolerances in the bolt are tightened up and the bolt from bolt face to locking lugs compress (not much but some). That tiny movement of the bolt face moving back away from the chamber allows the case head to follow causing the slight amount of stretch which in turn will eventually cause case separation.

Case stretch that causes head separation occurs near the case head not at the mouth. Some case stretch does occur in the shoulder and body area but that is caused by the flattening effect of pressure. Cases used with high pressure loads cause case stretch and require trimming. Incipient case separation is the impending separation near the case head and is not correctable, the case is history. Neither of which is related to chamber wall texture.

PJ spoke of cleaning cases, that is correct, cases of handloads should be cleaned. Not for the reasons Varmint Al cites, but because the case lube will build up in the chamber and cause case dents and hard extraction.

Clear as mud?
:tee:

 

There is an assumption that polishing the chamber will increase bolt thrust. Take this for instance polish your cases, polish you chamber both to a micro smooth finish. Now that sounds like a good idea making that chamber super slippery so you cases will thrust back. Unless there is something in between these two all you have done is increase the amount of surface contact which in our real world will actually increase the grab at top chamber pressure negating the desired theory.

I don't know how many of you folks reading this have ever used Johanson Blocks for precision tooling and building. They are standard measurement tools to make sure you are doing the same thing time after time, Cadillacs were the first to be built using these as a standard for manufacture. Their motto became "The Mark Of Excellence"

Now back to the polishing, these "Jo Blocks" are machined and highly polished, beyond what you can imagine, but if clean and placed together they will stick to each other and you can lift two at a time because of such total surface mating because of the surfaces being in full contact.

I do not know if this will happen during the chamber pressure spike or not easy to find out, polish your chamber but prior to doing it, check to see if your primers are sticking out any. Then check the primers again after polishing. If they are sticking out more then the case is sticking indicating less bolt thrust. If they are flatter that indicates increased bolt thrust. Have a great day sorting this out or a great day of shooting.

 

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Thanks Swany,

Interesting post. And yes, I know what the Jo blocks are. :biggrin:

 

@Swany - I see your point. I had overlooked the situation that two very polished surfaces will tend to "weld" under extreme pressure together better than two "not-so-polished". Naturally, I'm the guy who wants to put a lubricating film between case and walls that is pressure/heat tolerant, such as a moly coating.

@JBledsoe - You paint a clear picture, at least for me. I can "see" the headspace allowing the case to stretch at the head when the rest of the case expands to the chamber walls.

I've been recently observed totally lost from the cow trail, wandered about apparently aimlessly, and I am tentatively feeling my way about here. I wish to avoid more cow patties.

If I've followed you correctly, your position would be that no matter how magically friction-free I make the chamber/case interface, pressure on the bolt-face will remain as factory-intended, given repeatably-exact pressures from shot to shot. Case separation in a super-lubricated chamber will naturally occur when the brass at the case head (from repeated firings) has a cumulative stretch. That stretch would be caused by the case "filling" the distance between suddenly-flattened-back bolt (during peak pressure) and at-rest-unfired case head (before ignition).

BANG, and that gap is closed, primarily, you paint, by case stretch (more or less, there being other minor contributing factors such as primer and case movement while the bullet exits the case).

So, what I'm now pondering is a glued-in cartridge with a bit of headspace at the bolt. That bolt in theory should be able to withstand 100% of the psi generated by the ignition. Some of the psi will be absorbed by the case stretch, and in the event the case is glued into place, the only part of the brass that can stretch is the case head. That stretching will, sooner or later, bring the demise of the case.

What I'm also pondering is a case covered in "magic lube" that will be contained by the chamber and bolt face, but will be able to stretch as best it deems fit, (probably not as much at the case head as would the glued-in case) because the entire casing gets a bit bigger when free-floating.

Your picture paints, in either extreme event, pressures on the bolt face that would seem to be identical regardless of case/chamber affilliation.

PapaJohn might chime in here. Can anybody disagree with his "In a Marlin, extra bolt thrust is NOT a good thing."

But he might be able to defend "I don't want slick shells in my chamber, increasing bolt thrust."
Thrust, not pressure, is PapaJohn's focus.

I think his premise is that the ignition (an equal and opposite reaction) sending out the bullet is the same ignition thrusting back the case. Possibly a friction-contact with the chamber walls would reduce that backwards before the pressure ever touches the bolt face. PapaJohn is distinguishing between the pressure in the case as opposed to the thrust of the case itself slamming against the bolt.

Grips

 

Grips,

I suddenly think I got lost on the direction of your question, with the PapaJohn reference....

So maybe this will be irrelevant to what you are asking, sorry if that is the case.

Bolt thrust, chamber grip, and case head seperations are all inter-twined, but not single issues.
Generally speaking here is the cycle: BANG, bolt thrust as the cartridge moves back, then case grip against the chamber wall, bullet leaves and pressure drops, case grip on chamber releases, case shrinks(somewhat) so extraction is possible.

In order to be able to extract the case, you have to have a little room. When the bolt is closed and locked, in the case head area, there is a small gap(chamber to brass). This is the area where case head seperations happen. There is no "gripping" surface for the brass to grab. Also the brass gets stretched beyond spec. As that "extra" stretch happens, you get work hardening, AND that is also one of the hotter places in the case, so you get some flame erosion(pitting) happening.

As to the "extra bolt thrust is bad" comment. That is true for ANY gun, not singularly Marlin.
If you design a system for 60,000 PSI(example), Running that system @ 70,000 PSI has the consequence of a shorter working life. In the case of a gun, taking "shorter life" to it's logical extreme; leads you to a dental bill. If you catastrophically weaken the reciever lock-up area, or bolt lugs. When they fail, the bolt gets sent through your cheek. That is a bad day.

 

Chamber pressures do have a unique thing that in general they increase bolt thrust. Here is the exception the straighter the walls the less the thrust, this means a 30-30 AI can maintain higher pressures with less bolt thrust. That straight in line surface grips the chamber wall so tight that bolt thrust is negated. In fact a test was done that featured a standard 30-30 and a 30-30AI chambered in a bbl with no bolt but a firing device to ignite the primer, the standard backed up through a chrony at 2200fps, the AI stayed in the chamber. Given if you have an AI version most often you can exceed the pressures designed for the Lever action yet the amount of metal in the chamber area of a Marlin is sufficient to do this, albeit safely I don't know but assume so as it is done. With no bolt thrust at peak pressures who knows what could be accomplished then most AIs do out perform their legit parent.

 

I wonder if it's not the smooth chamber, but the ever-so-slightly increased chamber diameter that results from smoothing causing the increased bolt thrust?

I'd say the bolt thrust problem is a much bigger concern than case head separation (sorry, reloaders), since it affects the life of the gun and possibly the dentition of the shooter. Either way, I'm not messing with the chamber on ANY of my guns.

 

Good information flowing here, thank you.
My concern was to minimize the forces on the bolt. I might have mis-labelled this thread "chamber polishing". It properly should be called "chamber slicking."

So, for consistency of arguments, let's eliminate "polishing to make bigger" - which it might. Instead, let's just "fill the pores" with moly (or some high-tech nano-ceramic such as put out by Otis). For discussion purposes, we have not altered the chamber dimensions, just the effective surface friction on the walls.

So far in this discussion, there seems to be a consensus that psi alone, within factory limits, will not alter force on the bolt whether in a rough-walled or slick-walled chamber.

We have moved on to discussing how the thrust of the casing backwards alters the dynamics of how force is placed on the bolt. The bullet going thataway and the casing going thisaway slams the casing onto the bolt face.
That is the image I have in mind, at least.

Proponents of "not messin' with the chamber" contend that the normal BGMA (Before Grips Messin' About) chamber/casing shotgun wedding mitigates the casing's thrust on the bolt. VarmintAl's FEI suggests there is a measurable difference in thrust onto the bolt based on "slickness" or "roughness" of the chamber. PapaJohn voices a similar concern. I'm using these two only as token reference points to the "make it only un-rough-enough to extract the fired casing" camp.

Swany suggests that straight-walled chambers apply less thrust onto bolt faces than do tapered and/or bottle-neck chambers. Within his suggestion is the implication that there is *some sort* of chamber-to-case wall friction that is essential to the safety limits of thrust on the bolt (the back door to the high-pressure chamber).

Then I come in and start wondering what will happen if I paint moly on the chamber walls on my 444. Or moly-coat the casings for dry-lube in the tube and up the action.

So I'm askin', know'n that it's safer to make mistakes in here than on the range.

Grips

 

Here's my 2 cents. Case stretch is worsened with excess headspace. The firing pushes the case forward a bit as it fires, the case grips the chamber walls as it seals, and the rear of the case flows back against the bolt face, the cause of case stretch. A slick case/chamber will increase thrust against the bolt. How much, I don't know and will leave that part of the debate to the experts. I do know that in one instance oiled cases with reduced charges were used to fire form in order to correct for some severe excessive headspace in a Jap 99 7.7mm. (that rough as a cob gun had an extremely fine finished chamber) Gripping the chamber walls is not the same as sealing at the moment of firing. A rough chamber will grip the case at the moment of firing and normal headspace tolerances will increase case stretch with each FULL LENGTH re-sizing of the case, resulting in lower case life expectancy. This is why when reloading for the same rifle the FL sizing die should be backed off in order to not push back the shoulder. Straight walled cases are another matter in that regard, but with a rough chamber reloading will result in case stretch. I would think that a chamber wall surface finish of 32 or slightly better meets the needs of the modern reloader. AC

 

Hi Guys,
Had an issue at the range I want to run by yall. I have a Marlin 1894 .44 mag (bout a 2011 model). Custom cut back job to 16" and Gunkoted. Really slick nice gun. When I first got it back from 'smithy I purchased 300 rds of Herters 240 gr fodder from Cabelas. Mostly for the brass and to get familiar with the gun. IMMEDIATELY up on shooting the Herters I had FTE after FTE...I mean to the point the cases had to be pushed out. Hmmm...okay. I figured the cheap soft brass was expanding too much? After I accumulated enough (other manufacture) brass I began reloading 200 gr Lazer cast bullets over Titegroup. Worked like a champ, fed and shot accurate enough.

Initially I had some extractor issues, that with the help of this website I was able to poish and tune those away. Yesterday came the real issue: I was out doing some plinking and decided to check some heavier loads. I loaded up with Win white box 240 gr stuff. To extract the shell took considerable effort........this was a first for that particular load. I cam home and perused this site......
I polished the chamber and bolt face as described and feeding is slick as it gets. Have yet to shoot the heavy load again.

My question is this:

-Did the Herters brass expand too much because its soft/cheap? seems to operate fine out of my buddys Ruger pistol?
-Did the Win white box decide to stick "now" because of left over lead goo in the chamber from the 400-500 rds I shot thru it?
-Or was it a combo of the outside temp and all the above?

Any insight would be appreciated.

Thanks,

Beau
-

 

Hey Beau, Al from Florida here. I would start with a baseline clean gun, bore, and chamber. The old Herter's from the '60s was notorious for cheapness and low quality. (I'm sure a whole new ball game these days - hopefully good) Clean the chamber/bore and retest. 400-500 rounds rates a cleaning. Good thing you got that handy Takedown screw. AC

 

I didn't read everything...

If you want a slippery case/chamber, oil would accomplish that. Have you ever shot a rifle with a oiled chamber or a wet one? You get overpressure signs.

I have a US Model of 1917 that has a very rough chamber. One set of cartridges have over 50 firings on them and they are still going strong.

If you want to extend your case life, neck size only.

 

Thanks Al. I'll give it a shot!

 

New here but I am a varmint al reader also. His analysis is correct as far as I can judge. The FEA model does not care how the friction is established between the brass and the chamber ID steel. He set the friction parameter to model different scenarios using various coefficients of friction. I would venture that if there is lube (viscous liquid) material between the case and chamber wall (the cylindrical parts) then there will be a lower coefficient of friction than if the mating surfaces were in molecular contact with no intermediary lubricant.

As to the case being inside a sealed cavity at time of firing, that is not true, particularly for the .243 case that varmint al used for the model. Push fed, rimless, bottle necked cartridges are held in the forward position, shoulder against chamber conical transition, just before firing by the spring loaded ejector pin. Additional forward force is applied to the cartridge by the firing pin as it sinks into the primer cup during ignition. Typically there is .003 to .005 clearance between case head face and bolt face at ignition, depending on how much "case headspace" is set by the sizer/reloader preference or factory forming.

As varmint al's nonlinear dynamic model animation shows, the primer cup pressurizes and slides out the back of the case head primer pocket until it meets the bolt face. Then, as the internal pressure of the case rises toward max, the back of the case (case head) slides out the back and envelopes the primer cup again until the case head hits the bolt face.

Typically for the .308 family of bottle neck rounds (.243 included), about 0.187" of the case hangs out the back of the chamber. There is no radial support of chamber to case for this distance.* Longitudinal support of the case head is provided by the bolt face, finally, as the case head impacts the bolt face. As chamber pressure decays the ballooned case relaxes a little (hopefully) and the case head moves forward off of the bolt face, leaving a slight clearance. You can run your own experiment firing cases with no powder to see how the primer cup slips out the back and takes up the "case head space".

The cartridge designers do rely on radial force generated friction from the body of the case to the chamber wall to help grip and resist the longitudinal forces trying to stretch the case, and help reduce bolt face load.

There are some exceptions like the German roller lock autoloader designs that purposely want high pressure gas to flow between case and chamber wall. But, those bolts are engineered to take the thrust load, and the gas flow is timed to occur when the neck to chamber seal is broken after peak chamber pressure is reached and starts to decay.

*This is the reason that case head structural failures around or through the primer cup are typically disastrous. There is no pressure barrier even though the bolt is locked.

 

Methinks there is too much reading and not enough shooting going on here! :hmmmm:

 

Right on, Hal

As long as we stick with loading manual loads (which we should all do) all this is just "bar stool BS" and doesn't amount to a hill of dung in the real world. :biggrin:

 

The Mauser stg45 designed, HK developed and refined system uses the longitudinal chamber grooves to flow gas back around the case at the moment of firing in effect lubricating it because without them the design would have the bolt ripping the rims off the cartridge upon extraction. Kinda like the cartridge oiler system on old Jap machine guns from WWII to facilitate extraction. If you are so deep and financially involved and committed to a design you will do what it takes to make it work. Even if it defies conventional engineering wisdom. Look at Honda's Hybrids. They are recalling them to reconfigure the battery vs engine cycle with the vehicle's computer to save battery life and avoid battery warranty claims while losing their high mileage numbers (numbers used to make sales) in the process. Stick with what works. There is essentially nothing new out there , only 'UNREDISCOVERIES'. AC

 

It's an interesting topic, nonetheless. But let's look at a commonality in all brass cases when they are fired.

That commonality is this:

The back end of the case does not grip. Not with any normal pressure load, even that fired in a .270, with its high pressures. This is easily seen by looking at a fired case. Just how much the burnished part of the case grips is arguable; the 45 ACP or 38 Special case wall clearly do not get pressed as hard against the chamber walls as a .270's brass does. Then we can get into minutiae about time to peak pressure, fast versus slow powders, and all the other obfuscating folderol.

But the back of the case doesn't grip. Since it is free to move, obviously, what happens then?

It is free to thin, and prone to separate at the head, if it has the room and potential to move. At 38 Special pressures this is one thing. At 270 pressures it is quite another. And a springy action adds another variable. So simply stating that polishing the chamber adds anything to the ability to resist head separations is nonsensical. Other factors have a lot more to do with the issue that that......e.g. the mentioned pressures, and brass thickness, and headspace. And action spring.

The 38 Special revolver has enormous headspace issues......look at the gap between case head and recoil shield when the gun fires. Do these cases separate at the head? Absolutely not. Polishing the chamber won't have a darn thing to do with it......and a lot of this has to do with pressure and the yield strength of brass.

Take a look at a high pressure rifle cartridge. Polish the chamber all you want, but if action spring and headspace gaps and pressure allow it, the cartridge is gonna separate at the head, and sooner rather than later. Chambers are not cleaned as often as they could or should be, and this affects the grip issue.

Further, take another look at the "bolt thrust" debate with a more critical eye. If the head of the cartridge does not grip, and the brass thickness of the case wall is the weak link, the case will stretch back to meet the bolt even if it is straight walled, and this stretching will occur at lower than maximum pressures because the yield strength of brass is limited. If this is the case, and it often is in many "straighter walled" cases, then the reduction in bolt thrust may be reasonably questioned. How is the delivery of bolt thrust lessened if the case head moves?

Take a look at a straight walled cases versus a tapered in the "bolt thrust" issue. Given that action spring occurs in leverguns especially, and that action springing must occur at some level of pressure, it is reasonable to see that when springing does occur, a tapered case will resist easy extraction more so than a straight one........since we are wedging a straight "stretched" area into a more tapered chamber. It is probably the physical characteristics of the case itself that result in the "easy extraction" more so than any significant reduction in bolt thrust......since the straight case stretches in the head area to meet the bolt face just like a tapered one.

No less an authority than Bob Forker saw fit to question the reduced head thrust claims of a straight case. Brass thickness sees to it that bolt thrust delivered is similar in a straight or tapered case, but the difference in extraction seems to confuse the issue.

Try pounding a tapered peg with a short straight section in the rear in a tapered hole sometime. It sticks tight.

 

What also confuses the issue is the difference in capacity with an "Ackleyized" case versus a tapered one. The Ackley case has more capacity, and attains the same velocity with lower pressure. Lower pressure leads to less rearward slam and a lesser degree of stretching at the case head due to less action springing. Combine this with a straighter case with less "wedging" effect, and of course extraction is easier. But at an equivalent level of pressure, the force the bolt sees should be equivalent for a straight or tapered case. The interior diameter of the case head is still pretty much the same.

And the case head stretches to meet the bolt, even in an Ackleyized case.

.

 

"So far, it has been concluded that bottle neck cases such as .308 fare worse at the base for stretch than do most straight walled cases."

Grips, that's incorrect. Case shape has little to do with it. Headspace slack, whether occasioned by the rifle or the over sizing of cases, pressure, and action springing have more to do with case stretch than case shape. Thicker brass can stretch more before separating than thinner brass, but most brass designed for high pressures has a pretty thick amount of brass near the base.

 

"So far, it has been concluded that bottle neck cases such as .308 fare worse at the base for stretch than do most straight walled cases."

Grips, that's incorrect. Case shape has little to do with it. Headspace slack, whether occasioned by the rifle or the over sizing of cases, pressure, and action springing have more to do with case stretch than case shape. Thicker brass can stretch more before separating than thinner brass, but most brass designed for high pressures has a pretty thick amount of brass near the base.

Point taken. Do you have any objection to the following correction:
"So far, we have concluded that cases with unsupported brass and/or higher pressure within the brass (such as with .308 compared to .444) and/or weaker brass tend to have more failures."

Grips

 

.

Hey 35remington.

Correct on all counts, well stated.

:tee:

 

@35remington: You state an example: "Try pounding a tapered peg with a short straight section in the rear in a tapered hole sometime. It sticks tight. " Are you saying the ignition of the powder will pound the case in the muzzle direction? If not, what does this example have to do with firing a round in a chamber?

But if your example is to demonstrate how "stick" is greater in tapered chambers than straight, then your next post where you state "But at an equivalent level of pressure, the force the bolt sees should be equivalent for a straight or tapered case" is confusing. Are you saying that the tapered .444S chamber will tend to "stick" the cases more than would a straight wall, but that "stick" won't reduce "rearward slam" on the bolt face?
Grips