6.5 x 47 Lapua

[alinwa]

Which ones?

I'll reiterate two that have already been mentioned, #1, that the brass casehead can/will "recede" or draw back further than the boltface leaving the primer popped up and #2 that dragging weighted plates in any way equates to the interaction of the brass case to the chamber walls.



Everything deflects and springs back - the barrel, action, bolt and brass. The final primer protrusion is the product of all of these.

Of course, but anyone with even a basic understanding of springback realizes that they all exhibit different rates of same. I'll not waste time explaining the obvious differences. If you're not implicitly aware of the fact that the "lead slug deformation" of the casehead is different than the radial deformation and springback of the casebody I can't make you see it.




Are you saying that the brass in the casehead somehow violates the laws of physics?

No.......and therein lies the problem with VA's cartoons. It's the assumptions that are wrong, not the physics




.

I'm not going to go very far with this because I can't, politely. It's obvious you've never actually experienced primer popup, but if you do please ask yourself "WHY is the amount of protrusion directly related to the amount of headspace present?"

I'll not pick on Varmint Al, he simply doesn't model well. He misses the point most of the time.

But while you're at it, ask yourself "WHY can some setups show ZERO tendency toward casehead separation? Also ZERO case growth and ZERO primer popup, while others show one or all of these symptoms?" WHY can one guy fire a case 100 times at 70,000psi while the next guy has problems with moderate loads?"


Incidentally you're on the right track with the plastigauge but lead shot works even better. And gold foil makes a great gauge for comparing higher chamber pressures. And various tapes work well for establishing headspace testing parameters. And PLEASE DO do some testing!!


PLEASE!


It's the difference between reading a book and riding a bike.

 

[alinwa]

Well, I lied. I am gonna' pick on Varmint Al, a liddle.

Please note that my aim IS NOT to "pick on Varmint AL," it's to illustrate my point which is, PLEASE PEOPLE, DO NOT ASSUME that because someone says something it's automatically true!


This stuff just slips by....


I find VA really hard to read because an awful lot of stuff is just "slipped in" as assumption and unless one has actually BUILT STUFF one can be led down a rosy path. In his modeling Al consistently states as fact that more highly polished chambers are more slippery and that "rough chambers" grip the case such that they're more likely to stick in place.


Please, will some of you (Keith ) take the time to check on this?? All you've got to do is spend a few hours playing with the finish inside your chamber. I've done this. I don't find smooth chambers to be more slippery........


And Keith, I found VA's simulations to show primers popping up and then being squished back into place once, crumpling the corners. This matches my own observations. True "popped up" primers still have rounded edges...... and when you remove the primers and reinsert the empty cases THEY STILL HAVE THE SAME HEADSPACE GAP AS BEFORE FIRING.

 

[mks]

#1, that the brass casehead can/will "recede" or draw back further than the boltface leaving the primer popped up and #2 that dragging weighted plates in any way equates to the interaction of the brass case to the chamber walls.

Neither of these are assumptions. 1) The brass is modeled as an elastic/plastic material, which it is, and the recession of the case head is calculated by the laws of physics. 2) Al simulated a whole range of friction coefficients. Just pick the one you think is correct, and see if the simulated motion follows what you expect.

BTW, how would you quantify the friction coefficient? The weighted pull experiment is a straight-forward, low-budget approach, but its applicability to cartridge cases does depend on the linearity of the coefficient to higher loads. How would you do it?

Of course, but anyone with even a basic understanding of springback realizes that they all exhibit different rates of same. I'll not waste time explaining the obvious differences. If you're not implicitly aware of the fact that the "lead slug deformation" of the casehead is different than the radial deformation and springback of the casebody I can't make you see it.

Yes, of course, they have different rates. But to contend that brass acts like brass in the neck and body, but acts like lead in the case head, doesn't make sense. Caseheads deflect and spring back like brass, not like lead.

I'm not going to go very far with this because I can't, politely. It's obvious you've never actually experienced primer popup, but if you do please ask yourself "WHY is the amount of protrusion directly related to the amount of headspace present?"

Nah, we can be polite, cause this isn't personal, it's a discussion of the science. We are both looking for the truth, right?

There are two ways you can get primer protrusion: 1) Low pressure loads in which the casehead deflection is elastic, and comes short of the boltface. In this case the final casehead to boltface clearance (I think there are those who would argue against calling this headspace) is the same as the initial clearance, and the primer protrusion is proportional to that clearance. 2) High pressure loads in which casehead deflection is plastic. In this case, the final clearance is less than the initial clearance, and the primer protrusion is proportional to the final clearance, not the initial clearance.

But while you're at it, ask yourself "WHY can some setups show ZERO tendency toward casehead separation? Also ZERO case growth and ZERO primer popup, while others show one or all of these symptoms?" WHY can one guy fire a case 100 times at 70,000psi while the next guy has problems with moderate loads?"

By avoiding plastic deformation of the casehead, by using a stiff barrel, action and bolt, and low clearance. Your answer is the same, right?

The way we differ, it seems, is whether elastic deformation occurs. The unquestionable observation is that the case length is the same before and after firing. But what happens in between? Does it make sense that a case head exposed to 70 ksi doesn't move at all? I don't thinks so. It deflects and returns to the same length.

 

[alinwa]

Neither of these are assumptions. 1) The brass is modeled as an elastic/plastic material, which it is, and the recession of the case head is calculated by the laws of physics. 2) Al simulated a whole range of friction coefficients. Just pick the one you think is correct, and see if the simulated motion follows what you expect.

I don't think anything about the modeling is correct although the last movie is closer to what actually happens. BTW at no point do I see where the primer cup re-protrudes. I must have missed that movie?

BTW, how would you quantify the friction coefficient? The weighted pull experiment is a straight-forward, low-budget approach, but its applicability to cartridge cases does depend on the linearity of the coefficient to higher loads. How would you do it?

I wouldn't attempt to "quantify" it, knowing from prior work that there is no linear relationship, nor is there a similarity between a block and a pressure-supported flexible casewall. I'd work with the much simpler criteria of "slide"/"no-slide."



Yes, of course, they have different rates. But to contend that brass acts like brass in the neck and body, but acts like lead in the case head, doesn't make sense. Caseheads deflect and spring back like brass, not like lead.

No, the difference is simple math. My "lead slug" analogy was an attempt at shortcutting a lengthy explanation about the ratios involved. Stretching the tube several thou in length over a very short section of the case VS stretching it a thou or two over the entire radial hoop of the case



Nah, we can be polite, cause this isn't personal, it's a discussion of the science. We are both looking for the truth, right?

There are two ways you can get primer protrusion: 1) Low pressure loads in which the casehead deflection is elastic, and comes short of the boltface. In this case the final casehead to boltface clearance (I think there are those who would argue against calling this headspace) is the same as the initial clearance, and the primer protrusion is proportional to that clearance. 2) High pressure loads in which casehead deflection is plastic. In this case, the final clearance is less than the initial clearance, and the primer protrusion is proportional to the final clearance, not the initial clearance.


I agree



By avoiding plastic deformation of the casehead, by using a stiff barrel, action and bolt, and low clearance. Your answer is the same, right?

The way we differ, it seems, is whether elastic deformation occurs. The unquestionable observation is that the case length is the same before and after firing. But what happens in between? Does it make sense that a case head exposed to 70 ksi doesn't move at all? I don't thinks so. It deflects and returns to the same length.


Here we diverge rather widely..... I've never observed an instance, not ONCE in many years of measuring cases where "the case length is the same before and after firing." So calling that an "unquestionable observation" on your part puts us well into the realm of differing opinions and even perhaps beyond it into "Have you ever actually DONE this???" This is what I meant about the polite part. I have to question this statement of "fact" and in my experience when I do this folks get mad and walk off........But I can't imagine that anyone who's actually measured for expansion would have found "the same case length before and after firing."

I kinda' agree with your last part......YES the entire system deflects and YES part of the answer is to keep clearances small and YES this is to minimize plastic deformation.........but MINIMIZE....... not eliminate. Yes, one can eliminate the problem of casehead separation by eliminating the stress riser at the web but one cannot keep from having to resize the cases. The only thing we don't resize for is casehead expansion, which is different than that of the entire rest of the case. Not only is the ratio wildly skewed (expansion of a thick disc of copper VS the thin hoop of the casewalls) but the pressure acting to expand the casehead isn't even in the same room with the pressure acting on the main casebody. In fact, caseheads wouldn't expand measurably AT ALL if it wasn't for the primer pocket and flashhole.

But I digress

we'll try for the "answers in bold above"

 

[mks]

I don't think anything about the modeling is correct ...

I wouldn't attempt to "quantify" it (the friction coefficient)...

VarmintAl, if you are following this thread, I now understand why you haven't jumped into the discussion.
Alinwa, neither models nor experiments are perfect, but we can learn from both. By dismissing models, you are missing out on half of the pool of available knowledge.

I've never observed an instance, not ONCE in many years of measuring cases where "the case length is the same before and after firing." .

But from your previous post:

...the empty cases THEY STILL HAVE THE SAME HEADSPACE GAP AS BEFORE FIRING

It is the latter "observation" (yours) that I am not "questioning." If, in fact, they experienced no plastic deformation, this does not mean that they did not deform elastically. Regardless of how much plastic deformation occurs, spring back of the elastic part of the deformation provides a mechanism for primer protrusion to increase as the casehead recedes with the falling chamber pressure.

Cheers,
Keith

 

[alinwa]

VarmintAl, if you are following this thread, I now understand why you haven't jumped into the discussion.
Alinwa, neither models nor experiments are perfect, but we can learn from both. By dismissing models, you are missing out on half of the pool of available knowledge.



But from your previous post:


It is the latter "observation" (yours) that I am not "questioning." If, in fact, they experienced no plastic deformation, this does not mean that they did not deform elastically. Regardless of how much plastic deformation occurs, spring back of the elastic part of the deformation provides a mechanism for primer protrusion to increase as the casehead recedes with the falling chamber pressure.

Cheers,
Keith

I dismiss this modeling because it's not relevant. In simple terms, real world cases DO NOT exhibit characteristics as predicted by this "modeling" session. Cases do not stick any better in rough chambers and the statements that they DO are evidently the result of not testing real-world because in firing rounds in real rifles it quickly becomes apparent that chamber finish has little to do with it.

I was unclear...... my term "THEY STILL HAVE THE SAME HEADSPACE GAP AS BEFORE FIRING" is incorrect...... they still have NEARLY the same headspace gap as before firing. Even when bouncing back elastically the cases do grow some at each hit.