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Dumb question about neck clearance & expansion upon ignition

Very interesting thread. I think one challenge we face to understanding is comprehension of the velocities involved and the “quickness” of pressure effects. Discussion of “thin” necks above ties into the concept of gas pressure working its way between bullet and neck wall. Explosion velocity of the powder charge, and speed of the pressure front advancement, is MUCH faster than the acceleration of the bullet’s mass into the barrel. If we can conceive of the case walls blowing out to the chamber before the bullet fully leaves the mouth, and we know that neck expansion will stop the instant the pressure inside the case can escape outside the case mouth, then we know all of this is taking place in a very short period of time. The case sealing against the chamber walls is what would prevent carbon depositing all around the outside of the case. Since significant carbon is usually only found in that ideal sine wave pattern around the outside of the neck, the area below that carbon must have sealed off before gas escaped the case mouth. This effect could occur either by the gas escaping the instant the bullet base (or pressure ring) moved past the mouth, or by the peeling of the neck from the bullet by the gas “excavating” its way past the bullet as the bullet is accelerating.

Gas escapes around the bullet even after it is engraving, but there is still sufficient pressure behind the bullet and continues to build until the bullet is a short way into the lands (as @DaveTooley mentioned above) to accelerate the bullet to its muzzle velocity. So, its the initial and retained pressure behind the bullet that effects the bullet’s acceleration. The bases of my thinking on this are the relative velocities and time frames that MUST be occurring. Bullets are fast, but they are not traveling at 22,900 FPS (the speed of the gas/pressure front), especially as they start to accelerate. The entire time between powder ignition and the bullet moving past the mouth, the pressure inside the case is pressing on the brass. If that pressure can slam the thickest part of the case against the chamber wall before the gas can bypass it (after escaping the case mouth), then its ability to work through the weakest part of the system (the bullet/neck contact) and expand the weakest part of the brass (the thin neck) seems easily conceivable. Again, the bullet is moving exceedingly slowly at this point (relatively).
 
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.012" is like the perfect neck thickness in my opinion for larger cases (Above 22-250)
.009-.010" is good for smaller cases I have found
I have not tried thinner than this since in my belief I think it would contribute to split necks
I mean with less material present, the easier it will work harden right?
I mean maybe .007-.008 may be fine also
But lets say we take it down to half like in the .005" area
I believe necks would split much sooner
So, I don't know what they mean by "Thin to Win"
Maybe simply..... "Thinner" is a better way to word it
More importantly, the neck thickness being uniform all the way around the neck is a better way to think of it
which inherently, results in lighter tension.
Since this would allow for even release all the way around we have improved a variable to a constant.
---------------
I ALSO, definitely believe, that being thinner allows for a lighter squeeze or hold on the bullet
This thinner aspect, is one way of adjusting neck tension
Not to be confused with sized neck dia, which is irrelevent
We can have 2 different brands of brass (Lapua vs Federal) being the same neck thickness of say
.012" yet each one will have vastly different holds on the bullet
Therefore we can ascertain that thickness alone does not equate to equal neck TENSION. across the board for all brands of brass.
Lapua will have more tension being tougher more springy brass
Therefore, we may have to turn the Lapua brass down MORE to achieve the same desired neck HOLD
on the bullet, as we would have with leaving the Federal brass thicker.
---------------
The podcast guys may be hinting to something I have found for years
LIGHT neck tension provides me the best... most consitent... most accurate results
So they may be saying thin to win, not realizing or admitting a trade secret
That thin results in lighter neck tension (or lighter hold to be more accurate)
Light neck tension in my experience also greatly helps improve bullet concentricity
because there is less force fighting the bullet to go in and stay in straight
no spring back or deformation from the neck/shoulder junction etc
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The clearance to the chamber is not so much a big deal if the neck can expand enough to provide even bullet release all the way around its diameter.
once we have say .002" clearance per side , its all good,
of course more clearance does not hurt anything. so some people have more to ensure things.
----------------
I know many people use neck tension as a tuning method
I do not
Since if ALLL my neck tensions are the same (all light) that parameter is now even across the board
from one gun to another, they are all the same. (Light)
I have never once. NOT been able to make a gun shoot very accurately with light neck tension
including factory rifles with no mods.
so I dont see the whole using more neck tension to compensate for better load development
Load development and seating depth is where its at.
The perfect powder, the perfect primer and the right bullet for your barrel.
More tension would build pressure faster right? (peaking sooner)
You could simply bump the load up to achieve the same thing or use a different primer.
But I believe neck tension is a smaller contributor to accuracy albeit a contributor nonetheless.
---than proper load development
IF, a person uses load development instead, they may achieve the same thing as varying neck tension.
if someone is using something like HEAVY neck tension to tune their load, more power to them
it is a way, but maybe there is another way , that being powder charge or a powder with more desireable ignition characteristics
If you notice, many people will adhere faithfully for instance to using Varget
So they insist on forcing their gun to swallow Varget then run the gamut of using all kinds of methods to get it to shoot accurately
When perhaps, if they uniformed their neck tension to be light and remove it as a variable
and
Switched powders, they may achieve the same or in fact better results
Again, there are different ways to accomplish load development and each individual has their own ways
So - not suggesting varying neck tension is wrong since it obviously provides results
But stating instead
(What if we instead reduce the amount of Variables present to have to chase or tune?
It is easier for me to simply switch powders than to have to spend a bunch of money on special dies and a bunch of different sized bushings and test different neck tensions.
Note: (large calibers such as 300 WM and above with slow powders may be an exception to this theory liking more neck tension than the smaller calibers) They use heavy bullets and maybe they need all the help they can get getting initial pressures to spike to get the bullet dirving down the lands.
---------------
But, what if we have only 7 variables to tune instead of 10 variables?
Brass characteristics,(material composition), brass weight, brass neck thickness, brass neck tension, primer pocket uniformity and chamfer size, neck length,................
There's 6 brass variables right there and we havent even put primer powder or bullet yet
Which is why I state, I like to reduce to amount of variables and convert them to constants.
If all my neck tension is light, the same, can tell by feel off the bat if it needs to be annealed just by how the bullet seats....
I can focus on other variables instead since I know my brass will always be the same.
No matter if it is sized .002" or .004" in the neck.
(Dang, sorry for the novel here, stream of thought process lol)
I must iterate, Not suggesting I am right and somebody else is wrong, more than one way to skin a cat theory here, by all means experiment, have fun with it, play with neck tension if you like, I personally, prefer to remove that as a variable in the equation when there are so many variables to have to try and narrow down....again allowing for all neck tensions to remain in the light category
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(jamming vs jumping may be another way to replace using the variable of varying your neck tension)
such as, having light neck tension and jamming may accomplish the same thing as Heavy neck tension and jumping. with regards to initial powder ignition.
If you listen carefully, Jack is talking about doing what it takes to create enough loaded round clearance. Do you have one or more rifles with tight neck chambers?
 
A few years back when I was listening to a podcast where Cortina was interviewing Jack Neary. . . if I recall correctly, or maybe it was Speedy. Anyway, a term was mentioned briefly that I hadn't heard before: "thin and win"

It was about turning the necks thin, which I understood would increase the clearance. I don't quite understand why "thin" is a good idea as I've not heard any details about it. Like, is it that it quickens the necks expansion and sealing the chamber? Or is it the extra clearance in the chamber? Or is it the lighter "neck tension"? Or, is it just all of the above? Anyway, I thought I'd try it in my .308 and trimmed some necks down to .012" and gave them a try. HUH . . . wouldn't you know, sure enough the results were actually pretty good. I was actually expecting worse than I usually get for my SAAMI spec chamber.
That's what I was told by Pat at PMA tools when I had to turn some brass to fit by new chamber. I thought .012" seemed to thin, but it does just fine.
 
That's what I was told by Pat at PMA tools when I had to turn some brass to fit by new chamber. I thought .012" seemed to thin, but it does just fine.
Barrels with tight neck chambers almost always have the neck diameter engraved on the barrel. Do you know who chambered your barrel? If you do, you could ask the gunsmith about his reamer's dimensions.
 
Probably a dumb question, but I'm trying to understand what happens to the neck diameter upon firing and why you need neck clearance.

Upon ignition, the pressure inside the case expands the brass to form to the chamber, but the neck which contains the bullet doesn't see that pressure because the base of the bullet contains the pressure below it. When the pressure is enough to push the bullet past the neck the pressure would expand the neck, but if that's the case would you need neck clearance at all? If the neck was a 'zero clearance' situation to the chamber, wouldn't the bullet be propelled forward the same, the neck just wouldn't be expanded by the pressure below it like in a normal chamber with clearance after the bullet starts its travel down the barrel? (I understand if there is less than 'zero clearance' that pressure would skyrocket). Does the bullet need the neck to expand to move forward, or does the neck expand from pressure after the bullet has moved forward?

And then, is neck clearance actually necessary, or is it just a safety margin in case you have case neck thickness variance that would create a pinched neck dangerous situation?
In short, if you don’t have some clearance at the neck, you will collapse the case behind the neck.

The air trapped between the case and chamber walls needs a place to go. Same thing happens if the shoulder seals at the wrong time.

This can also be a problem with low pressure loads so there is a balance. I have subsonic loads that after firing, the bullet is a slip fit in the neck, except a small roll on the mouth. Just not enough pressure quick enough to blow it all out. And it stops when the gasses find an easier path than expanding the neck while pushing the bullet. More frequent when the bullet completely leaves the case before it hits the lands.

Strait wall cases have the problem of case collapse, since there is no shoulder or defined neck. Full peak pressure gets between the case and chamber. One solution is to bell the case mouth to seal the chamber. Preventing the collapse.

As others have stated, no clearance would raise starting pressure, effectively changing the entire curve. Just like the amount of crimp changes the pressure curve, sometimes to a disastrous effect.

In general, carbon on the neck, no big deal, carbon below the shoulder, needs a look as to why.

Remember the process from primer ignition to case expansion is measured in milliseconds, probably should be microseconds. When the bullet is released completely from the neck, due to movement or case expansion changes with bullet bearing surface and jump or jam, plus pressure and speed of combustion. There is no absolute answer.
 
This can also be a problem with low pressure loads so there is a balance. I have subsonic loads that after firing, the bullet is a slip fit in the neck, except a small roll on the mouth. Just not enough pressure quick enough to blow it all out.
Answers a question I have long wondered about.
 
Barrels with tight neck chambers almost always have the neck diameter engraved on the barrel. Do you know who chambered your barrel? If you do, you could ask the gunsmith about his reamer's dimensions.
My 30-06 is a 337 instead of a 340 so I knew I needed to turn them. But in my 6.5 Creedmoor I bought some 6.5 Creedmoor Peterson Fat Brass that I did not know that is what I had. So I was getting pressure signs then finally figured out the problem and just wanted to turn them down to .014 and Pat at PMA told me to do .012 but I tried .014' and it was not enough. So I called him and asked if I could send the cutter back so he could make it into .012. He did but, I had to buy him lunch. Well worth the lesson. So now after that I'm turning some Starline brass to .012. After reading this forum I just turned a batch, couldn't help myself Going to turn the 200 pieces I just got to work up some loads for my new Lapua 144gr. OTM bullets.IMG_1067.JPG
 
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I have yet to see a definitive measure of when the bullet leaves the neck and engraves into the bore. Many folks agree that the bullet can start moving simply from the primer and I have experienced that before. But, it takes a lot more pressure to fully engrave. A general assumption is well above 2000psi. For a .22 bullet that's about 80lb of force so something I would not disagree with.

It takes a significant amount of pressure to expand the case neck. A SWAG would be around 1000 to 2000 psi depending on caliber and neck thickness. Thinner neck means easier to expand (which may be where the BR guys favor thinner necks). Simple hoop stress.

Combine those two and it means the neck should seal the chamber by the time the bullet is sealed in the bore. Given how little the bullet needs to travel it means there will be little time for 'blow by', either around the neck or around the bullet. The small area for 'venting' means a 'choke point' that will still make pressure build rapidly. Which also creates the blow by on the necks.

Bottom line is the bullet will begin moving before the neck starts to expand, but, part of it will still be in the neck when the neck seals the chamber. Thinner necks make that happen faster, as would seating the bullets out to jam into the rifling (higher start pressure).

Keep in mind this is my opinion based on a SWAG and is no means a well done analysis (me in my recliner :) ).

FWIW, I'd love to see some analysis of initial bullet movement vs chamber pressure. I know in cannons they have used laser radar to measure the projectile movement in the bore. Not a cheap thing to do so not sure if anyone has done it in a rifle barrel before.
 
.012" is like the perfect neck thickness in my opinion for larger cases (Above 22-250)
.009-.010" is good for smaller cases I have found
I have not tried thinner than this since in my belief I think it would contribute to split necks
I mean with less material present, the easier it will work harden right?
I mean maybe .007-.008 may be fine also
But lets say we take it down to half like in the .005" area
I believe necks would split much sooner
So, I don't know what they mean by "Thin to Win"
Maybe simply..... "Thinner" is a better way to word it
More importantly, the neck thickness being uniform all the way around the neck is a better way to think of it
which inherently, results in lighter tension.
Since this would allow for even release all the way around we have improved a variable to a constant.
Given identical chamber dimensions, going thinner will work that part of the brass more (it's moving more when it expands and the resized back down).

I've typically turned my necks to .014" and it's worked fine for me, as I've done that to have uniform neck thickness. When I turned some down to .012" thickness, obviously they're going to have to expand more in my chamber when they seal off. Though I anneal after ever firing, I've just not heard of any results of extended use for thinly turned necks used in no-turn chambers.

---------------
I ALSO, definitely believe, that being thinner allows for a lighter squeeze or hold on the bullet
This thinner aspect, is one way of adjusting neck tension
Not to be confused with sized neck dia, which is irrelevent
We can have 2 different brands of brass (Lapua vs Federal) being the same neck thickness of say
.012" yet each one will have vastly different holds on the bullet
Therefore we can ascertain that thickness alone does not equate to equal neck TENSION. across the board for all brands of brass.
Lapua will have more tension being tougher more springy brass
Therefore, we may have to turn the Lapua brass down MORE to achieve the same desired neck HOLD
on the bullet, as we would have with leaving the Federal brass thicker.
Certainly, the compressive stress changes with the difference in neck thickness and the particular elasticity for the make up of the brass alloy. A combination of this along with frictional interference all affects how the bullet is being released.

One of the things I found fascinating in the Houston Warehouse Test, when it talked about loading the cartridges where the bullet was loaded long with almost no interference allowing the bullet seating to be set when the pushed into the lands. The cartridges would be loaded long and let the lands push the bullet further into the case. Not what you'd want when feeding from a mag. ;)

---------------
The podcast guys may be hinting to something I have found for years
LIGHT neck tension provides me the best... most consitent... most accurate results
So they may be saying thin to win, not realizing or admitting a trade secret
That thin results in lighter neck tension (or lighter hold to be more accurate)
Light neck tension in my experience also greatly helps improve bullet concentricity
because there is less force fighting the bullet to go in and stay in straight
no spring back or deformation from the neck/shoulder junction etc
I tend to like light "neck tension" as well. Though, if I'm feeding from a mag, the tension I use probably not a good thing. . . especially when I'm loading to a mag length with short bullets.

----------------
The clearance to the chamber is not so much a big deal if the neck can expand enough to provide even bullet release all the way around its diameter.
once we have say .002" clearance per side , its all good,
of course more clearance does not hurt anything. so some people have more to ensure things.
----------------
I know many people use neck tension as a tuning method
I do not
Since if ALLL my neck tensions are the same (all light) that parameter is now even across the board
from one gun to another, they are all the same. (Light)
I have never once. NOT been able to make a gun shoot very accurately with light neck tension
including factory rifles with no mods.
so I dont see the whole using more neck tension to compensate for better load development
Load development and seating depth is where its at.
Yes, I too feel seating depth is really important. . . especially consistent seating depth. And neck tension can be a big factor in getting that.

The perfect powder, the perfect primer and the right bullet for your barrel.
More tension would build pressure faster right? (peaking sooner)
You could simply bump the load up to achieve the same thing or use a different primer.
But I believe neck tension is a smaller contributor to accuracy albeit a contributor nonetheless.
---than proper load development
IF, a person uses load development instead, they may achieve the same thing as varying neck tension.
if someone is using something like HEAVY neck tension to tune their load, more power to them
it is a way, but maybe there is another way , that being powder charge or a powder with more desireable ignition characteristics
If you notice, many people will adhere faithfully for instance to using Varget
So they insist on forcing their gun to swallow Varget then run the gamut of using all kinds of methods to get it to shoot accurately
When perhaps, if they uniformed their neck tension to be light and remove it as a variable
and
Switched powders, they may achieve the same or in fact better results
Again, there are different ways to accomplish load development and each individual has their own ways
So - not suggesting varying neck tension is wrong since it obviously provides results
But stating instead
(What if we instead reduce the amount of Variables present to have to chase or tune?
It is easier for me to simply switch powders than to have to spend a bunch of money on special dies and a bunch of different sized bushings and test different neck tensions.
Note: (large calibers such as 300 WM and above with slow powders may be an exception to this theory liking more neck tension than the smaller calibers) They use heavy bullets and maybe they need all the help they can get getting initial pressures to spike to get the bullet dirving down the lands.
---------------
But, what if we have only 7 variables to tune instead of 10 variables?
Brass characteristics,(material composition), brass weight, brass neck thickness, brass neck tension, primer pocket uniformity and chamfer size, neck length,................
There's 6 brass variables right there and we havent even put primer powder or bullet yet
Which is why I state, I like to reduce to amount of variables and convert them to constants.
If all my neck tension is light, the same, can tell by feel off the bat if it needs to be annealed just by how the bullet seats....
I can focus on other variables instead since I know my brass will always be the same.
No matter if it is sized .002" or .004" in the neck.
(Dang, sorry for the novel here, stream of thought process lol)
I must iterate, Not suggesting I am right and somebody else is wrong, more than one way to skin a cat theory here, by all means experiment, have fun with it, play with neck tension if you like, I personally, prefer to remove that as a variable in the equation when there are so many variables to have to try and narrow down....again allowing for all neck tensions to remain in the light category
-------
(jamming vs jumping may be another way to replace using the variable of varying your neck tension)
such as, having light neck tension and jamming may accomplish the same thing as Heavy neck tension and jumping. with regards to initial powder ignition.
Though I'm not a bench rest competitor, addressing/eliminating as many factors as I can helps me focus on those things I had less or not control over. Some of them don't make a difference for my skill of shooting, but since I have the time, I do like to remove as many variables as I can.

If the neck must seal off the pressure, before the bullets enters the rifling
how do we get carbon on the necks and shoulder if the neck has sealed and takes such little pressure to expand?
---------
not being a wise ass here, just asking
there was this same discussion some time ago recent where a few engineers mathematically pertaining to pressure formulas and brass elasticity gave compelling results as to why the neck does not expand until the bullet has left the neck
-------------
I would have thought myself however---
The neck would slowly roll out expanding progressively just behind the base of the bullet as the bullet progresses out of the neck.
---as opposed to most peoples thinking of the whole neck all at once SUDDENLY expanding.
which makes no sense
meaning, if the bullet is halfway out of the neck, how is that portion of the neck that is behind the bullet... ....still remaining at bullet dia. if pressure is now being exerted upon the inside of the brass neck.
and why has not the portion behind the base expanded..... if it will expand once the bullet is suddenly
ALL THE WAY out of the neck?
----------
I was argued wrong there too by some compelling math which basically showed
the bullet must be out of the way for the gas to push against the inside wall of the neck
it should not be leaking past the bullet neck seal to expand the neck
because it can only expand if gas molecules can exert pressure on the inside of the brass wall
which requires the bullet to leave
----------
I still hold to the fact :
the neck should unroll progressively as the bullet leaves and gets out of the way
once the base has left the neck, there should now be a complete neck to chamber seal
if this is so, is the carbon on the necks perhaps sneaking by the neck to the shoulder as the brass is contracting once the bullet has left the barrel?
I think yes
if we have a weak load, the brass will not have as much pressure exerted to the chamber wall
and may barely seal, yet stil seal of course, since as you say ....don't take much
but will contract back down sooner since pressure was already low and allow gas to reverse back
------
IE; with more pressure the brass should stay expanded long enough for all carbon/gas to clear out
so I believe it is not a brass sealing question with regards to carbon on the shoulder
but more a correct pressure solution
I find it strange that some have said----
To remedy carbon on the shoulder, MORE neck tension cures it???
How would MORE tension trying to squeeze itself down......provide a BETTER OUTWARD seal against the chamber?
-------
BUT- with more neck tension, this would Drive pressure UP, UP enough to do as I suggest
keep the brass expanded longer
Hmmm??? you bring up something I hadn't really thought about before. As the pressure in the chamber decreases, when does the neck's springback allow some of those gases to flow back towards the shoulder, if at all?

I don't know how one would identify whether powder deposits were deposited before or after the sealing of the neck???

That's what I was told by Pat at PMA tools when I had to turn some brass to fit by new chamber. I thought .012" seemed to thin, but it does just fine.
I can see that for a tight chamber where turning in needed, that .012 would be just fine as long as there's enough clearance. From a metallurgical POV, it wouldn't be any kind of issue, except in some loose factory not turn chambers, the brass would get more of a workout with the cycles of firing and sizing reducing their life spans.
 
Just gonna add that I see no reason, for whatever the brass sees, to be less than peak chamber pressure, wherever it occurs, before bullet exit. It seems implied, to me at least, that the case only feels pressure while the bullet is in the case. Rather, I'd suggest it feels peak pressure as well as any pressures while the bullet is still in the bore.
 
Just gonna add that I see no reason, for whatever the brass sees, to be less than peak chamber pressure, wherever it occurs, before bullet exit. It seems implied, to me at least, that the case only feels pressure while the bullet is in the case. Rather, I'd suggest it feels peak pressure as well as any pressures while the bullet is still in the bore.
This only applies if gas pressure is equal in the entire pressure chamber, that pressure chamber being bolt face/case head to bullet base. I’ve never seen a gas that can be two pressures in the same container.
 
If you have Quickload or I assume use GRT use the pressure over bullet travel graphs.

Set your barrel length to cartridge length. What this does is produce a graph showing length of travel as bullet length.

If these graphs can be trusted, you can plot where the bullet starts and finishes being engraved. Quickload uses 3626 psi as the pressure developed to unseat the bullet in the neck and start the engraving process. Max pressure is generally reached once the bearing surface has been completely engraved and no more bullet compression is taking place. This makes perfect sense as the bullet has the least friction, and the pressure chamber/barrel volume increases as the bullet moves down the barrel.
 
This only applies if gas pressure is equal in the entire pressure chamber, that pressure chamber being bolt face/case head to bullet base. I’ve never seen a gas that can be two pressures in the same container.
Right, except its actually case id rather than case head. At least that's how bolt thrust is calculated. So, my point was that the case feels peak chamber pressure...the whole case. Because peak pressure happens 3 or 4 inches down the bore, not the case mouth. This also disproves a lot of thought regarding bullet jump vs jamb. Its not a guess, as I've seen it with a pressure trace system, firsthand and you can google pressure trace graphs yourself. Its not a judgement call or my calculations. Full bullet engraving happens before peak pressure occurs, in every single example I've seen.
 
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Right, except its actually case I'd rather than case head. At least that's how bolt thrust is calculated. So, my point was that the case feels peak chamber pressure...the whole case. Because peak pressure happens 3 or 4 inches down the bore, not the case mouth. This also disproves a lot of thought regarding bullet jump vs jamb. Its not a guess, as I've seen it with a pressure trace system, firsthand and you can google pressure trace graphs yourself. Its not a judgement call or my calculations. Full bullet engraving happens before peak pressure occurs, in every single example I've seen.
Thats why I qualified the use of the quick load graphs with “if you can trust”.

The reason I used case head and or bolt face is that we were talking gas pressure, if you have gas pressure on the bolt face you have other problems. Bolt thrust pressure will vary with case diameter when gas pressure is the same.

The there is the problem of mid pressure cartridges that don’t stretch enough to hit the bolt face, those that leave the primer proud. Do they also experience peak pressure at the bolt face?

Where low pressure cartridges don’t have enough pressure for the case to grip the walls will hit the bolt face with full pressure.

It’s going on in a very short time.
 
I have noticed a difference at longer ranges that "thin to win" is valid. I just shoot my turned neck brass that's at .012 in Starline or other brass that needs to be turned for one reason or another. Not my Lapua brass. I do not care if the turned brass does not last as long because it is more consistent and for me that does matter at 1000 + yards, the only place I use it. The brass is expendable like every thing that goes boom. . I use a different powder in that brass to get more velocity, RL16 instead of IMR 4350. I can get more velocity and do better in the wind, but it does come at a price. Brass life. Lapua weighs 167grs., Peterson-165grs, Starline-163.2 grs. and Hornady-154.5grs..
 
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Thats why I qualified the use of the quick load graphs with “if you can trust”.

The reason I used case head and or bolt face is that we were talking gas pressure, if you have gas pressure on the bolt face you have other problems. Bolt thrust pressure will vary with case diameter when gas pressure is the same.

The there is the problem of mid pressure cartridges that don’t stretch enough to hit the bolt face, those that leave the primer proud. Do they also experience peak pressure at the bolt face?

Where low pressure cartridges don’t have enough pressure for the case to grip the walls will hit the bolt face with full pressure.

It’s going on in a very short time.
Bolt thrust varies with case diameter. Pressure does not. For normal loads the gas pressure inside the cast acts over the area of the bolt face. Any 'leakage' is insignificant.

When pressures get low enough the case effects get weird. Lots of leakage and the combustion by products make things really dirty. Nothing significant when it comes to the strength of the the gun.
 
.015 for 35 years, turned necks and no reason to change. Lots of theroy not so much valid testing. I agree, thin to win. Problem is shooters from varying disciplines thrown in the same pot of soup.
 
Thats why I qualified the use of the quick load graphs with “if you can trust”.

The reason I used case head and or bolt face is that we were talking gas pressure, if you have gas pressure on the bolt face you have other problems. Bolt thrust pressure will vary with case diameter when gas pressure is the same.

The there is the problem of mid pressure cartridges that don’t stretch enough to hit the bolt face, those that leave the primer proud. Do they also experience peak pressure at the bolt face?

Where low pressure cartridges don’t have enough pressure for the case to grip the walls will hit the bolt face with full pressure.

It’s going on in a very short time.
My point is that the entire case feels peak pressure, from case mouth to base. You mentioned case head to "bullet base"...but peak pressures happen after the bullet is well clear of the case mouth.

From your comment below, I take it you feel that the neck doesn't feel pressure while the bullet is still in the neck. The carbon sine wave seen on necks is evidence that gas leaked by before the neck fully seals. But if that happened while any portion of the bullet is still in the neck, that's pretty clear evidence that the neck does indeed see pressure. If it leaks between bullet and neck, the neck gets pressurized. How/why else would there be carbon to get on the neck? Without the neck sealing to the chamber, the case would likely collapse when the bullet exits the neck.
I think the way you're looking at it would be true IF the neck was very rigid and unable to yield. But it's not. I still think my leaky hose analogy in a previous post is accurate in this context. The case neck is more similar to a rubber hose than it is to a very rigid fixture that can contain the pressure without yielding. Similar to a hose clamp on a ..rubber hose. Granted, very different pressures but it's just an analogy. Again, how does fluid under pressure leak by a barbed fitting that extends inside of the hose and tightening the hose clamp stops the leak? I'm saying the hose yields, allowing the leak and that tightening the clamp makes it a rigid fixture that stops the yield...and hence, the leak. The case neck is similar to the leaky rubber hose UNTIL it contacts the rigid chamber wall and seals there.
This only applies if gas pressure is equal in the entire pressure chamber, that pressure chamber being bolt face/case head to bullet base. I’ve never seen a gas that can be two pressures in the same container.
 
My point is that the entire case feels peak pressure, from case mouth to base. You mentioned case head to "bullet base"...but peak pressures happen after the bullet is well clear of the case mouth.

From your comment below, I take it you feel that the neck doesn't feel pressure while the bullet is still in the neck. The carbon sine wave seen on necks is evidence that gas leaked by before the neck fully seals. But if that happened while any portion of the bullet is still in the neck, that's pretty clear evidence that the neck does indeed see pressure. If it leaks between bullet and neck, the neck gets pressurized. How/why else would there be carbon to get on the neck? Without the neck sealing to the chamber, the case would likely collapse when the bullet exits the neck.
I think the way you're looking at it would be true IF the neck was very rigid and unable to yield. But it's not. I still think my leaky hose analogy in a previous post is accurate in this context. The case neck is more similar to a rubber hose than it is to a very rigid fixture that can contain the pressure without yielding. Similar to a hose clamp on a ..rubber hose. Granted, very different pressures but it's just an analogy. Again, how does fluid under pressure leak by a barbed fitting that extends inside of the hose and tightening the hose clamp stops the leak? I'm saying the hose yields, allowing the leak and that tightening the clamp makes it a rigid fixture that stops the yield...and hence, the leak. The case neck is similar to the leaky rubber hose UNTIL it contacts the rigid chamber wall and seals there.
I agree with all of what you said, but need to clearing up the case head to bullet base comment. In a perfect world, no leaks, all gas would be in the case supported by the chamber, the bore and contained by the bullet base. I have referred to the bullet as the pressure relief valve. Nothing else should move. Case expansion is the exception but that is a single event. Unless the case fails. If there was no bullet, there would be relatively no pressure.
 
I agree with all of what you said, but need to clearing up the case head to bullet base comment. In a perfect world, no leaks, all gas would be in the case supported by the chamber, the bore and contained by the bullet base. I have referred to the bullet as the pressure relief valve. Nothing else should move. Case expansion is the exception but that is a single event. Unless the case fails. If there was no bullet, there would be relatively no pressure.
Exactly! But what we have here is not a "perfect world" scenario, where all the pressure stays behind the bullet base, because the neck yields....err expands like a rubber hose. As soon as it leaks, it sees pressure like the rest of the case. If not, the neck wouldn't seal and the case would likely collapse or at least be very dirty from possibly equalized pressures inside and outside the case.
 

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