Dumb question about neck clearance & expansion upon ignition

[slm9s]

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?

 

[243winxb]

Light for caliber bullets, 68 gr, in 243 win, using IMR4350 or 4831, starting loads willl not expand brass necks to the chamber. This is a factory chamber. No annealing of brass.

Neck clearance, diameter, is just a safety margin. As is shorter then needed trim Length.

https://www.varmintal.com/arelo.htm#Fitted_Necks

FITTED NECK CASES


Lots more great info from Varmint Al

 

[joshb]

How perfect are your measurements? I try to leave 4 thousandths clearance in my turned neck guns. That’s a wee bit. Am I a perfect turner? NO!
It’s a safety issue to avoid a pinched neck.

 

[243winxb]

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

I think yes.
I think its all done to align bullet to chamber. Plus, anytime gas blowby can be decreased, barrels will last longer & produce better accuracy.

My unsized part of the neck is the same diameter as my factory neck chamber, after 3 or more firings of full pressure loads.

Bullet alignment to the chamber is done by
1. Bullet Jam.
2.Tight neck Chamber & minimum shoulder setback.
3. Sizing only part of tne neck.

 

[gunsandgunsmithing]

If the neck doesn't see pressure until the bullet leaves the neck, would there still be gasses in front of the bullet or is that 100% leaking by the bullet, already in the bore? I'd suggest that some leaks by the neck/bullet and once that happens, the neck sees pressure.

 

[ELR LVR]

You pose some good theory
and I agree, if the pressure is pushing the bullet out of the neck while the neck is staying the same dia
and only expands AFTER the bullet leaves.....why do we need neck clearance?
I mean heck, we stuffed that bullet in there without much more than 50 lbs of effort to begin with right?
it should be able to push it back out easily enough with all that 60,000 psi of pressure the gunpowder builds up???
Except if there is ANY restriction other than the bullets total mass, that pressure has to go somewhere if it cannot push the bullet smoothly, usually that pressure ends up in your face.
Soooooooooooo
--------------------
One reason is
1. Temp differences change diameters of things, especially between 2 dissimilar metals that don't expand at the same rate
2. Different bullets have different dia. pressure rings
Some have none while some have .0004"
so a .224 bullet actually measures .2244" dia at the pressure ring
so if you had whats called a line fit, (no clearance) the chamber neck would actually be squeezing the brass and driving pressures up since it cannot do what?
EXPAND
so now we need at least .001" clearance just for the pressure ring
then there is the fact nobody uses mics and instead uses calipers which are not truly accurate to .001"
---------
so lets say you measure .250" neck dia of a loaded round
and you have your chamber cut to a line fit of .250" neck dia
Well guess what? Your calipers can easily be off by .001" and actually your neck may be .251"
again, squeezing the neck driving up pressures to an unsafe level

So we need to build in some form of insurance so we don't drive pressures up by contracting down on the neck of the case.
Then we also have to figure in there is the fact of dirt, debris, carbon, (you ever see the floating debris in your room as the sunlight comes through your window)? Yep its true that stuff actually has a diameter to it
so we need to allow for some atospheric, debris, carbon, build up,inconsistencies with the neck dia which look at it through a microscope, is not perfectly flat all the way down and will have lines and ridges etc like the surface of a pavement road
so how much clearance is safe?
----------------
Some benchresters, will spec out their neck clearance to where it may expand just barely enough to allow bullet release but then contract back down to previous size so as to not have to resize the case.
They have done it, and it works
IF
all your brass is perfect, and cut, and turned, and measured not with calipers but with .0001" micrometers
which means,
Factory brass is never allowed
which means, you will never be able to buy factory ammo for a factory rifle and have it chamber unless there is some neck clearance as well as chamber clearance.
Believe it or not, the brass has to actually be allowed to balloon out for things to work in harmony.
------------------
So we factor in,
1. atmospheric debris and dust floating into the chamber ...(unless you shoot guns in a medical grade clean room with a sticky floor before you walk in)
2. carbon buildup
3. different brass neck thicknesses between manufacturers which can easily differ .002" or more.
4. different bullet pressure ring diameters
5. the fact diameters will quickly change once a gun heats up or is suddenly contracted BELOW original cut diameter when fired in the cold of winter during hunting season
=
and come up with at least .0047" neck clearance (or .00235" per side) to be reasonably safe and not hurt you, or the brass or most imprtantly
your rifle.
-------------------
THEN....what if your bullet is not seated perfectly concentric to 0.000" indicator runout
if the bullet is in the brass crooked, whats that mean?
YES it will oblong or geometrically ovalize the outside diameter of your brass neck which
Makes it bigger in one dimension above chamber neck diameter.
so dang, we better add in another thou for that
so lets round it all off to a decent
.003" clearance per side unless all you do with that gun is Benchrest in a clean environment with perfectly uniformed and neck turned and blueprinted brass
then maybe you could cut it down to .002" per side
--------------------
I will say I tried this recently with an experimental wildcat project I'm conducting some independant reserach on and thought I would test out this same theory you are posing here
---- a loaded round measures .250" at the neck with a bullet in the case
and I cut the neck at .2504"
its been working, loads and extracts fine, actually shoots very accurately too
BUT
I get these occasional flyers sometimes an inch away from the group
and my theory is, if any of the above mentioned criteria interfere with the neck chamber fit
That will obviously change pressures and likely is the reason for my flyers
so then after I am done with the current research I am doing, I will relieve the neck diameter to have more clearance and see if the flyers go away.
--------------------
After all...this game is all about accuracy, not just spitting bullets out the front
That's why we have neck clearance
Because- more neck clearance is not a bad thing
---too little of it is
--------------------
On a serious note here: I will say I think many of us benchresters got a little too happy with the whole
tight tolerance thing and incorrectly transferred the idea to our brass/chamber fit. Some of my most accurate guns are cut with Saami Spec reamers that I would swear are just too big to keep the brass aligned nicely enough to be accurate.
Like: I got a deal on a Carbide 22-250 reamer and when i got it and spec'd it out, I was kinda disappointed to find the neck diameter to have .014" neck clearance
or .007" per side right?
But DANG does that freaking gun shoot, and I mean effortlessly without trying = prints small groups.
I like guns you dont even have to try to put bullets in the same hole
I hate guns where you have to hold it just perfect and learn the personality of the rifle to find out just how it "likes" to be held for it to cooperate, (Half my guns are like that, I have to jot down notes in my log books of how each rifle must be held, some firm, some soft, some medium, some so tight your arm goes to sleep, only maybe 2 of them I can free recoil all the way up to a tight hard hold and it dont matter"
so if MORE neck clearance is part of the answer for a gun shooting well without trying,
I have to rethink my whole theory of having brass fit tight to the chamber. Or at least the neck clearance part of it.
Most of my reamers now I like .004" clearance in every dimension to allow for smooth and effortless chambering AND extraction

 

[Big Mc]

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?

The only dumb question is the one not asked. Tommy Mc You are fine Sir.

 

[Rank Amateur]

Just for consideration, I might suggest the answer may not be either nor or. Rather, the bullet may be moving AND the neck is expanding at the same time. The explosive velocities of smokeless powder components (typically nitrocellulose or nitrocellulose & nitroglycerine) are somewhere above 7 thousand meters per second (Nitrocellulose ~ 7,050 M/s; nitroglycerine ~ 7,700 M/s). So, the pressure inside the case rises from atmospheric to tens of thousands of pounds per sq in, essentially instantly. Pressure acts against all surfaces equally, regardless of shape. I would envision the walls of the brass case start moving out toward the chamber walls the instant they feel this pressure increase, and it wouldn’t surprise me if the cartridge walls are fully blown out to the chamber walls before the bullet leaves the case (see below).

Pressure will always also seek the easiest/weakest path, which is probably the weak friction junction between the bullet and the inside of the case neck. That pressure is trying to push past the bullet, instantly. As it moves past the base of the bullet and between the bullet and case neck the pressure in that miniscule space is ALSO at the same tens of thousands of pounds per sq in felt everywhere inside the case. So, as it is likely that the pressure pushing against the base of the bullet is sufficient to overcome the friction holding the bullet in the neck (so the bullet starts moving forward), at the very same time, the case neck is being “peeled” from around the bullet by the invading gas/pressure and blown toward the chamber walls. I.E., it may be that the bullet starts moving before the entire neck is expanded (at all), and the neck is being blown out and away from the bullet sides at the very same time. Even in cases where the chamber neck is reamed to “match” the OD of the cartridge neck, there will be some tiny clearance, and that clearance is sufficient for bypass of tens of thousands of pounds of gas pressure per sq in. Perhaps the blow out diameter and the spring back amount are exactly the same (so the fired cartridge neck diameter never seems to change, but I would submit that it DOES change sufficiently for the gas pressure to work it’s way between the bullet and the case neck.

What would not happen, though, is for the neck to expand at all once the bullet has left the case mouth. At that point, the pressure acting inside the case would almost instantaneously equalize with the pressure in front (and around) the case neck - at which point there would be no pressure differential expanding the case neck. As such, any and all neck expansion we observe must occur before the bullet actually fully leaves the case mouth.

Just my thoughts, would love to hear reaction and reasoning.

 

[BoydAllen]

In the case of neck clearance, a long time back. when fitted necks were in common use, some good shooters found that having clearance that required sizing shot better. Probably a quarter century back I discussed this with a world record holder that told me that he had been told that the reason that having more clearance worked better was that it gave a more uniform bullet release. More recently I have been told that this difference is easier to see at 200. Most recently, Jack Neary has used the expression, "go thin to win" which is entirely about clearance. My friend who owns so many records once told me that he tries for around two and three quarter thousandths clearance.

 

[jelenko]

the walls of the brass case start moving out toward the chamber walls the instant they feel this pressure increase, and it wouldn’t surprise me if the cartridge walls are fully blown out to the chamber walls before the bullet leaves the case (see below).

This^^

 

[Straightshooter1]

You pose some good theory
and I agree, if the pressure is pushing the bullet out of the neck while the neck is staying the same dia
and only expands AFTER the bullet leaves.....why do we need neck clearance?
I mean heck, we stuffed that bullet in there without much more than 50 lbs of effort to begin with right?
it should be able to push it back out easily enough with all that 60,000 psi of pressure the gunpowder builds up???
Except if there is ANY restriction other than the bullets total mass, that pressure has to go somewhere if it cannot push the bullet smoothly, usually that pressure ends up in your face.
Soooooooooooo
--------------------
One reason is
1. Temp differences change diameters of things, especially between 2 dissimilar metals that don't expand at the same rate
2. Different bullets have different dia. pressure rings
Some have none while some have .0004"
so a .224 bullet actually measures .2244" dia at the pressure ring
so if you had whats called a line fit, (no clearance) the chamber neck would actually be squeezing the brass and driving pressures up since it cannot do what?
EXPAND
so now we need at least .001" clearance just for the pressure ring
then there is the fact nobody uses mics and instead uses calipers which are not truly accurate to .001"
---------
so lets say you measure .250" neck dia of a loaded round
and you have your chamber cut to a line fit of .250" neck dia
Well guess what? Your calipers can easily be off by .001" and actually your neck may be .251"
again, squeezing the neck driving up pressures to an unsafe level

So we need to build in some form of insurance so we don't drive pressures up by contracting down on the neck of the case.
Then we also have to figure in there is the fact of dirt, debris, carbon, (you ever see the floating debris in your room as the sunlight comes through your window)? Yep its true that stuff actually has a diameter to it
so we need to allow for some atospheric, debris, carbon, build up,inconsistencies with the neck dia which look at it through a microscope, is not perfectly flat all the way down and will have lines and ridges etc like the surface of a pavement road
so how much clearance is safe?
----------------
Some benchresters, will spec out their neck clearance to where it may expand just barely enough to allow bullet release but then contract back down to previous size so as to not have to resize the case.
They have done it, and it works
IF
all your brass is perfect, and cut, and turned, and measured not with calipers but with .0001" micrometers
which means,
Factory brass is never allowed
which means, you will never be able to buy factory ammo for a factory rifle and have it chamber unless there is some neck clearance as well as chamber clearance.
Believe it or not, the brass has to actually be allowed to balloon out for things to work in harmony.
------------------
So we factor in,
1. atmospheric debris and dust floating into the chamber ...(unless you shoot guns in a medical grade clean room with a sticky floor before you walk in)
2. carbon buildup
3. different brass neck thicknesses between manufacturers which can easily differ .002" or more.
4. different bullet pressure ring diameters
5. the fact diameters will quickly change once a gun heats up or is suddenly contracted BELOW original cut diameter when fired in the cold of winter during hunting season
=
and come up with at least .0047" neck clearance (or .00235" per side) to be reasonably safe and not hurt you, or the brass or most imprtantly
your rifle.
-------------------
THEN....what if your bullet is not seated perfectly concentric to 0.000" indicator runout
if the bullet is in the brass crooked, whats that mean?
YES it will oblong or geometrically ovalize the outside diameter of your brass neck which
Makes it bigger in one dimension above chamber neck diameter.
so dang, we better add in another thou for that
so lets round it all off to a decent
.003" clearance per side unless all you do with that gun is Benchrest in a clean environment with perfectly uniformed and neck turned and blueprinted brass
then maybe you could cut it down to .002" per side
--------------------
I will say I tried this recently with an experimental wildcat project I'm conducting some independant reserach on and thought I would test out this same theory you are posing here
---- a loaded round measures .250" at the neck with a bullet in the case
and I cut the neck at .2504"
its been working, loads and extracts fine, actually shoots very accurately too
BUT
I get these occasional flyers sometimes an inch away from the group
and my theory is, if any of the above mentioned criteria interfere with the neck chamber fit
That will obviously change pressures and likely is the reason for my flyers
so then after I am done with the current research I am doing, I will relieve the neck diameter to have more clearance and see if the flyers go away.
--------------------
After all...this game is all about accuracy, not just spitting bullets out the front
That's why we have neck clearance
Because- more neck clearance is not a bad thing
---too little of it is
--------------------
On a serious note here: I will say I think many of us benchresters got a little too happy with the whole
tight tolerance thing and incorrectly transferred the idea to our brass/chamber fit. Some of my most accurate guns are cut with Saami Spec reamers that I would swear are just too big to keep the brass aligned nicely enough to be accurate.
Like: I got a deal on a Carbide 22-250 reamer and when i got it and spec'd it out, I was kinda disappointed to find the neck diameter to have .014" neck clearance
or .007" per side right?
But DANG does that freaking gun shoot, and I mean effortlessly without trying = prints small groups.
I like guns you dont even have to try to put bullets in the same hole
I hate guns where you have to hold it just perfect and learn the personality of the rifle to find out just how it "likes" to be held for it to cooperate, (Half my guns are like that, I have to jot down notes in my log books of how each rifle must be held, some firm, some soft, some medium, some so tight your arm goes to sleep, only maybe 2 of them I can free recoil all the way up to a tight hard hold and it dont matter"
so if MORE neck clearance is part of the answer for a gun shooting well without trying,
I have to rethink my whole theory of having brass fit tight to the chamber. Or at least the neck clearance part of it.
Most of my reamers now I like .004" clearance in every dimension to allow for smooth and effortless chambering AND extraction

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.

 

[BoydAllen]

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.

When Neary is talking it is to advise shooters how to get the most out of a tight neck 6PPC. Serious competitors who consistently do well in matches test everything extensively. Different situations may require different solutions. What do your loaded necks measure over the base of your bullets, and what do your fired necks measure at the same place? The difference, usually plus an allowance of .001 for spring back of unannealed cases, is your approximate loaded round clearance in your chamber. In the case of tight necked chambers, necks have to be turned for rounds to be able to be chambered. You can turn the necks any thickness you like. The question is how much clearance gives the best result. For the SAAMI chambers that I have loaded for, unturned necks provided plenty of clearance compared to what is used for tight neck chambers.

 

[Straightshooter1]

When Neary is talking it is to advise shooters how to get the most out of a tight neck 6PPC. Serious competitors who consistently do well in matches test everything extensively. Different situations may require different solutions. What do your loaded necks measure over the base of your bullets, and what do your fired necks measure at the same place? The difference, usually plus an allowance of .001 for spring back of unannealed cases, is your approximate loaded round clearance in your chamber. In the case of tight necked chambers, necks have to be turned for rounds to be able to be chambered. You can turn the necks any thickness you like. The question is how much clearance gives the best result. For the SAAMI chambers that I have loaded for, unturned necks provided plenty of clearance compared to what is used for tight neck chambers.

All very good points, and thanks for pointing them out. And I get all that.

What struck me at the time was there wasn't any suggestion or hint that "thin to win" was referencing to clearance in tight necked chambers or about any particular clearances. It seemed to be more about the way the neck expands when thin. The discussion didn't go into detail as I was all ears to hear more, but no joy. As you say, it does simply come down to just what gives the best results.

 

[BoydAllen]

All very good points, and thanks for pointing them out. And I get all that.

What struck me at the time was there wasn't any suggestion or hint that "thin to win" was referencing to clearance in tight necked chambers or about any particular clearances. It seemed to be more about the way the neck expands when thin. The discussion didn't go into detail as I was all ears to hear more, but no joy. As you say, it does simply come down to just what gives the best results.

Have you watched all of Jack's videos on this site?

Vihtavuori Powders

Share your videos with friends, family, and the world

www.youtube.com

As well as these?

All Things Benchrest with Friends!

All Things Benchrest with Friends.... Harley Baker, Jeff Summers and Jack Neary We each discuss every facet of short-range Benchrest Shooting and share some stories and laughs, too!

www.youtube.com

You may notice that pretty much all of it relates to short range group benchrest and one caliber.

 

[Straightshooter1]

Have you watched all of Jack's videos on this site?

Vihtavuori Powders

Share your videos with friends, family, and the world

www.youtube.com

As well as these?

All Things Benchrest with Friends!

All Things Benchrest with Friends.... Harley Baker, Jeff Summers and Jack Neary We each discuss every facet of short-range Benchrest Shooting and share some stories and laughs, too!

www.youtube.com

You may notice that pretty much all of it relates to short range group benchrest and one caliber.

Yes, I've seen all of that presentation shown on that Vihtavuori link. And I do recall that part in Part 2 regarding clearance. It was a similar discussion, but short, he had with Cortina's long podcast. I've not ever see the All Things Benchrest with Friends and will certainly take a look and see what I can glean from it all. Thanks.

 

[ELR LVR]

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.

.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
----------------
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
-------
(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.

 

[big5ifty]

Is your chambering for a turned neck or not ?

Neck clearance is not an adjustable factor for no-turn chambers, but uniforming the case neck thickness may help .

If there is zero clearance, the bullet is not released on powder ignition.

Anything that may happen under those conditions is beyond the design spec for smokeless powder rifles and rounds.

All the hardware is designed specifically for the neck to release the bullet on ignition.

 

[DaveTooley]

Consider that peak pressure is reached 3"-4" down the barrel. So it takes much less chamber pressure than we think to expand the neck. It may be that the neck expands prior to the bullet entering the barrel. Powder is ignited/pressure starts to builds/neck expands/ then the bullet enters the barrel. I think the neck has to seal off before pressure quickly increases as the bullet enters the barrel. Consider the much lower pressures in a revolver. The case does a pretty good job of sealing off the gases even at those much lower pressures.

 

[ELR LVR]

Consider that peak pressure is reached 3"-4" down the barrel. So it takes much less chamber pressure than we think to expand the neck. It may be that the neck expands prior to the bullet entering the barrel. Powder is ignited/pressure starts to builds/neck expands/ then the bullet enters the barrel. I think the neck has to seal off before pressure quickly increases as the bullet enters the barrel. Consider the much lower pressures in a revolver. The case does a pretty good job of sealing off the gases even at those much lower pressures.

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?
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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
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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?
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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
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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
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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?
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BUT- with more neck tension, this would Drive pressure UP, UP enough to do as I suggest
keep the brass expanded longer

 

[gunsandgunsmithing]

Ever had a fuel or hydraulic hose leak at the ends? How on earth did fluid get past the barbed nipple being inside of the hose. Why does it need a clamp at all? The explanations I recall from the engineers mentioned above, don't address that. Isn't this very much the same as a bullet seated in the neck?