Dumb question about neck clearance & expansion upon ignition

[Steve Donlon]

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.


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.


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.


Yes, I too feel seating depth is really important. . . especially consistent seating depth. And neck tension can be a big factor in getting that.


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.


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???


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.

I agree with what you said. By turning my necks to .012 my necks are the same as Hornady brass but more consistent. They seem to last okay but not as good as thicker brass. I was looking at Sierra load data and they use Hornady brass for their data. We get more velocity with Hornady brass and accuracy is not an issue for me, but I'm not a bench rest shooter with different expectations. So I will see what happens when pushed with RL16 powder and how long the turned brass lasts.

 

[Straightshooter1]

I agree with what you said. By turning my necks to .012 my necks are the same as Hornady brass but more consistent. They seem to last okay but not as good as thicker brass. I was looking at Sierra load data and they use Hornady brass for their data. We get more velocity with Hornady brass and accuracy is not an issue for me, but I'm not a bench rest shooter with different expectations. So I will see what happens when pushed with RL16 powder and how long the turned brass lasts.

Hmmm??? Interesting. . . that Hornady brass would be that thin. All my .308 and 6.5 PRC Hornady brass is close to the same as my various other brass (about a hair thicker), except I did just happen to get hold of some .308 Hornady Defense Black nickel plated that are much thinner than the others (like at .0135"). Hornady must produce thinner brass for your particular cartridge???

 

[Steve Donlon]

Hmmm??? Interesting. . . that Hornady brass would be that thin. All my .308 and 6.5 PRC Hornady brass is close to the same as my various other brass (about a hair thicker), except I did just happen to get hold of some .308 Hornady Defense Black nickel plated that are much thinner than the others (like at .0135"). Hornady must produce thinner brass for your particular cartridge???

Ya you're right. I just measured the whole box and some are almost.013"- .012" and a few, a little more in some sections. Sort of all over the place in my sample of 6.5 Creedmoor. That will be .007" clearance for my turned brass, which I know is not optimal, but from what I have read, doable. Consistency out weighs the increased clearance, I hope. I just checked my other 100 un-turned pieces of Starline brass and it is at .014"-.015" so I will compare the two and see what happens at 1000 yards for me. I know I'm not going to shoot 1/4 moa, but sometimes a I get a real good group at 1000 yards that has 6 shots stacked on top of each other. I was told it could be neck tension problems. That is what I need to fix.

 

[Edd]

How much pressure does it take to expand the neck?

How much pressure does it take to push the bullet into the lands?

How much pressure does it take to make the bullet move in the neck?

How far does the bullet move before it contacts the lands?

 

[ELR LVR]

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.


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.


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.


Yes, I too feel seating depth is really important. . . especially consistent seating depth. And neck tension can be a big factor in getting that.


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???


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.

Your line of thinking is logical and you got it figured out man
Yes when I am referring to 0.012" neck thickness that is taking into consideration most of us have custom guns with custom chambers which we usually have at least slightly tighter neck clearances
PRECISELY for the reason you stated,,,,to reduce the amount of brass fatigue that happens during the resize process
For instance : most of us want to at the very minimum "clean up the neck" on the brass
or take a skim cut
If we have something like .007" clearance on a factory chamber that would make for even worse neck clearance and more brass expansion right?
So yeah I am suggesting more for somethign with even slighhtly tighter chambers than a big sloppy factory one
---------------
I am ALSO though, speaking in general when a person does call out for a tight neck chamber
that staying in the .012" range would be a good idea for the arge cases
Meaning I would not want my LARGE cartridge TIGHT neck chamber require me to have to turn my LARGE caliber brass down to .010" like my 22BR brass.
I would not want to have to turn my 300WM brass down to .010" just to have neck clearance
--------------
And yeah, with regards to when the carbon gets deposited on the neck during the firing cycle
It is too hard to tell if it happens before the neck expands or after when gas pressure is still in the barrel and the neck begins to contract
But judging from the amount of gas still present say from a suppressed weapon, that gases and pressure do still remain for a moment even after the bullet has left the bore
which is why gas blocks are turned down to compensate so the rifle doesnt cycle like hell
compared to without a suppressor.
So my contention is that if the neck can expand so easily, and likely be sealing as the bullet is leaving the neck, and engaging the rifling while there is still some amount of bullet still in the neck
there should be no gasses leaking past the neck at this moment in time
UNLESS - you have so much freebore the bullet does not engage the rifling until the bullet has COMPLETELY left the neck
most of us have little freebore or are within .030" off the lands if off the lands at all
-------
Liked your post!!!
John
Make sense?

 

[ELR LVR]

How much pressure does it take to expand the neck?

How much pressure does it take to push the bullet into the lands?

How much pressure does it take to make the bullet move in the neck?

How far does the bullet move before it contacts the lands?

1. not much according to one poster here, and I agree
likely the same amount to squeeze it back down when resizing
you could use a spring compressor gauge press
2. more than the amount of pressure it takes to spit the bullet out the neck
3. -50 lbs of vertical pressure as --- I have tested this on a spring gauge with 22BR brass
----1 pound of pressure required from my RCBS press handle (again checked with trigger pull scale
but still retains bullet if extracting a loaded round if it has engaged the rifling
meaning the rifling will not pull the bullet out of the brass if removing case from chamber
more pressure for 300WM Brass which could be 3x more as I remember
4. You set this parameter with seating depth by taking a measurement where the lands are and start at jam and work backward until you find the best seat depth for accuracy
could be anywhere from -.120 off to .001 off (typically)

 

[mikecr]

I've run fitted necks, and thru testing, turning necks for greater clearance made no difference to MV.
By fitted, I mean <1/2thou clearance, so that fired necks do not have to be sized for reloading.
I have also tested release with bullets and necks dry polished with Tungsten Disulfide to cause very little seating force and finger removal of seated bullets. Again, no difference to MV over higher seating force conditions.

I believe that necks expand the billionth of an inch needed to completely release bullets, and that it takes very little pressure for that to happen. Hell, the shock impulse traveling up the case is likely enough for this.
The bullet is not preventing neck expansion.

I also believe that if we relied on pushing bullets out of necks, nothing about internal ballistics would be the same. That much containment might not even work.