A question regarding the "theory" of neck tension....

[griffgil1]

I have a more theoretical question: why is neck tension determined and stated in terms of the O.D. of the case neck, instead of the I.D.? It seems to me that the I.D. would be the more precise and accurate number, considering that it is the inside surface of the neck that contacts the bullet and "grips" it. Also, the math to get the I.D. is simple: O.D. minus 2x the wall thickness. I have been having real problems figuring out the proper neck tension, and I think it is because I'm not getting why all of the methods of figuring it involve measuring O.D. in some way. If my bullet has a diameter of .307, why not figure neck tension based on I.D., and know that it should be x thousands less than the bullet diameter? I am sure that I'm missing something (probably a very obvious something).

 

[TitaniumEdge]

For me, I can't accurately measure ID. Using a caliper with flat jaws for ID will give false reading, only real way to measure a round ID is with plug gauges, which most of us don't have

 

[DanConzo]

griff- Measuring the id directly is more accurate than relying on a varying wall thickness. IMHO.

 

[Link]

They measure the outside for people like me.

K.I.S.S.

;D

 

[ReedG]

O.D. is, as stated above, much easier and less expensive to determine. It is still a fairly precise gauge of the I.D. if you have consistent necks. It all relates to the diameter of the chamber neck and is really just a relative number of what works best in a particular chamber.

All of the common resizing dies determine the O.D. of the neck and "push" any imperfections in the brass to the inside. This is the main reason I am such a devotee of the Lee Collet Die which, of course, sizes the I.D. of the neck.

When we measure the neck O.D. to determine neck tension, we are, in fact, measuring the O.D. minus the neck thickness, to determine the I.D.

Given a .308" bullet with a case neck thickness of .012", if you want .002" neck tension it requires an O.D. of .330" (just using common numbers).

Bullet diameter: .308
Neck thickness: .012 x 2 = .024
.332
Minus desired tension: .002
Target O.D. .330

Hope that makes sense.

 

[Raythemanroe]

I would like to mention I think or agree that more time is spent on the outside even in neck turning! I have always thought that reaming and finishing the inside just as important and most posts I read are out side neck turn only!

 

[jlow]

It’s a real good question! Warning, if you think you have super accurate/precise neck tension, don’t read this….

If you are interested in figuring ID of a case or more specifically “neck tension”, try an interesting experiment. Take some cases that you have neck turned (or not for that matter), measure neck thickness with an accurate ball micrometer in three locations to get a good idea of average neck thickness. Now do the same for neck OD, measure it in three locations with a good micrometer to get an average OD. Calculate ID as you say ID = OD-(neck thickness x 2). Now do one more step which is to calculate bullet hold which is for a 308 = 0.308-ID.

To start this running, I will give you some scary numbers and stats from my own last reload for ten 308 rounds. This is Lapua brass that has been neck turned and neck tension adjusted with a Lee Collet die.

Average neck thickness = 0.01414”
Neck thickness ES = 0.00037”
Neck thickness SDEV = 0.00013”
Neck thickness ES as a percentage of average neck thickness = 2.59%

Average neck OD = 0.33398”
Neck OD ES = 0.00062”
Neck OD SDEV = 0.00018”
Neck OD ES as a percentage of average neck OD = 0.18%

Calculated average neck ID = 0.30570”
Calculated ID ES = 0.00097”
Calculated ID SDEV = 0.00033”
Calculated ID ES as a percentage of average calculated ID = 0.32%

So far so good. Notice that my percentage variance in all three never exceed 3%. Now for the bad part…

Calculated average bullet hold = 0.00230”
Bullet hold ES = 0.00097”
Bullet hold SDEV = 0.00033”
Bullet old ES as a percentage of average bullet hold = 41.97%

To see why that % number is so bad, if you look at the span of bullet hold, it range from a high of 0.00283” to a low of 0.00187”.

So why such ugly numbers when things seemed so rosy in the beginning?

First, that last number is the aggregate of three errors – two neck thickness and one neck OD. I’ll add them up for your (0.00037”x2) + 0.00062” = 0.00136”. So numbers that are consistently under one thousands are now above it….

Second, if you assume that we are aiming at a neck tension of 2.3 thousands but we are varying it by 1.36 thousands (59.13%), you can quickly see why we are in a bad place….

 

[watercam]

In the end all the theory in the world is great to play with. As far as shooting it seems that consistency is what makes accurate shooting possible. No one cares how anyone else determines their neck tension, pick a system and stick to it. When you finish your experimentation phase you can enjoy your shooting skills phase and have fun AT THE RANGE!
For me, I always measure directly to determine neck tension. If a loaded round (using brass prepped the way I like it) measures X, I subtract .001" (for single loading) or .003" (for magazine loading) from X and use neck bushings measured in those numbers. KISS.
No, I do not calculate neck tension based upon theoretical calculations. Yes, I buy at least three bushings before hand (they are not expensive and one never knows when one might want to try a different one).

 

[lpreddick]

this term has always confused me because the number is really neck diameter expansion. bushing sized a neck at .247, seat a bullet and measure neck diameter at .249 and we say we have .002 "neck tension". this measurement will probably stay the same for years, BUT tension, meaning "grip" on the bullet changes and my experience is that this "grip" stays uniform for awhile then increases. then there is the thing about "cold bonding/welding" where bullets are REALLY "griped" and pop if seated deeper. i wonder if this might be happening...size a neck, seat a bullet and there is an immediate "grip" on the bullet. as this case rest, the tension of the bullet stretching the brass continues until an equilibrium is reached ( minutes, hours, days?) and all is quiet. over time, the "grip" increases (why?) reaching another equilibrium. all the while neck diameter stays the same. this process probably varies based on the brass's condition...annealed vs work hardened vs brand vs thickness... an interesting and perplexing process.

 

[Raythemanroe]

It’s a real good question! Warning, if you think you have super accurate/precise neck tension, don’t read this….

If you are interested in figuring ID of a case or more specifically “neck tension”, try an interesting experiment. Take some cases that you have neck turned (or not for that matter), measure neck thickness with an accurate ball micrometer in three locations to get a good idea of average neck thickness. Now do the same for neck OD, measure it in three locations with a good micrometer to get an average OD. Calculate ID as you say ID = OD-(neck thickness x 2). Now do one more step which is to calculate bullet hold which is for a 308 = 0.308-ID.

To start this running, I will give you some scary numbers and stats from my own last reload for ten 308 rounds. This is Lapua brass that has been neck turned and neck tension adjusted with a Lee Collet die.

Average neck thickness = 0.01414”
Neck thickness ES = 0.00037”
Neck thickness SDEV = 0.00013”
Neck thickness ES as a percentage of average neck thickness = 2.59%

Average neck OD = 0.33398”
Neck OD ES = 0.00062”
Neck OD SDEV = 0.00018”
Neck OD ES as a percentage of average neck OD = 0.18%

Calculated average neck ID = 0.30570”
Calculated ID ES = 0.00097”
Calculated ID SDEV = 0.00033”
Calculated ID ES as a percentage of average calculated ID = 0.32%

So far so good. Notice that my percentage variance in all three never exceed 3%. Now for the bad part…

Calculated average bullet hold = 0.00230”
Bullet hold ES = 0.00097”
Bullet hold SDEV = 0.00033”
Bullet old ES as a percentage of average bullet hold = 41.97%

To see why that % number is so bad, if you look at the span of bullet hold, it range from a high of 0.00283” to a low of 0.00187”.

So why such ugly numbers when things seemed so rosy in the beginning?

First, that last number is the aggregate of three errors – two neck thickness and one neck OD. I’ll add them up for your (0.00037”x2) + 0.00062” = 0.00136”. So numbers that are consistently under one thousands are now above it….

Second, if you assume that we are aiming at a neck tension of 2.3 thousands but we are varying it by 1.36 thousands (59.13%), you can quickly see why we are in a bad place….

After all of that, we are assuming that our brass is going to have the same neck tension even if we got it right! I read the guy that set a new 1000k record using a King built rifle and I can't remember his name was annealing every time! I wonder if this helped with maybe controlling neck tension and helping reduce the vertical at 100k!

 

[ReedG]

Ray, I'm not much of a perfectionist and just try find something that makes sense to me and I can see results on the target. The only thing that matters to me is what the target shows. I use K&M neck-turning tools and run all the cases over the expander, which to some extent trues the inside of the neck and pushes imperfections to the outside where the cutter takes them off. From that point, I use Lee Collet Dies for all my calibers except the 30 BR (and I'm getting one made for that) and never look back.

Jlow: IMHO the piece of brass that can be accurately turned or measured to six digits beyond the decimal is in somebody's imagination. My world ends at "tenths" (.0000) and even then a "half-tenth" is fine with me. I'm just a "point-blank" shooter, and last time at the range I shot in a twitchy 12-14 mph wind, mostly from 7-8:00. How in the world would I be able to tell what one-hundredth-of-a-thousandth was doing? Then after I pull the trigger, everything has changed: the brass has aged, the tensile strength of the neck has changed, etc. If all those numbers work for you, that's the name of the game. I'm not capable of that level of technology and that's OK, too.

 

[M14AMU]

I have a more theoretical question: why is neck tension determined and stated in terms of the O.D. of the case neck, instead of the I.D.? It seems to me that the I.D. would be the more precise and accurate number, considering that it is the inside surface of the neck that contacts the bullet and "grips" it. Also, the math to get the I.D. is simple: O.D. minus 2x the wall thickness. I have been having real problems figuring out the proper neck tension, and I think it is because I'm not getting why all of the methods of figuring it involve measuring O.D. in some way. If my bullet has a diameter of .307, why not figure neck tension based on I.D., and know that it should be x thousands less than the bullet diameter? I am sure that I'm missing something (probably a very obvious something).

http://riflemansjournal.blogspot.com/2009/10/reloading-neck-tension.html

 

[Raythemanroe]

Ray, I'm not much of a perfectionist and just try find something that makes sense to me and I can see results on the target. The only thing that matters to me is what the target shows. I use K&M neck-turning tools and run all the cases over the expander, which to some extent trues the inside of the neck and pushes imperfections to the outside where the cutter takes them off. From that point, I use Lee Collet Dies for all my calibers except the 30 BR (and I'm getting one made for that) and never look back.

Jlow: IMHO the piece of brass that can be accurately turned or measured to six digits beyond the decimal is in somebody's imagination. My world ends at "tenths" (.0000) and even then a "half-tenth" is fine with me. I'm just a "point-blank" shooter, and last time at the range I shot in a twitchy 12-14 mph wind, mostly from 7-8:00. How in the world would I be able to tell what one-hundredth-of-a-thousandth was doing? Then after I pull the trigger, everything has changed: the brass has aged, the tensile strength of the neck has changed, etc. If all those numbers work for you, that's the name of the game. I'm not capable of that level of technology and that's OK, too.

I agree, Thats why I was wondering about annealing every time. I wonder If some one found a level in the brass field?

 

[jlow]

Watercam - Well I am also all for caring about one holer at the target, but to get there, you got to sweat the details, if you don’t then how do you know which of a 20-30 things you do when you reload are or are not consistent which in the end is what makes all the rounds go to that one hole? The answer is of course you measure and measure – I know this bugs some folks here but coming from the scientific world, I can assure you that there is no other way. The reality is what you describe i.e “measure x, I subtract 0.001” is in fact completely theoretical since you never measured the end results but just assumes……

Lpreddick – Agreed with you about the term “neck tension”, I personally don’t agree that it is an accurate descriptive term but just using here for descriptive sake. I am too afraid to get that “cold bonding” thing started up again…. :

Raythemanroe – I in fact anneal after every firing….

ReedG – I only gave 5 digits not six and for what it’s worth, the Mito micrometers can in fact measure accurately to 4 digits i.e. 0.0001” that last number is in fact a theoretical average and does not change my findings and conclusion. I stand by this statement.

 

[DanConzo]

I think ipredick is correct on the real definition of neck tension in this accuracy game. IMHO

 

[griffgil1]

ALLRIGHTYTHEN! Didn't intend to open a can of worms with this thread- but hey, the more I read the more I learn!
So, a "standard" method for determining proper bushing size, and thereby neck tension, seems to be subtracting .001" to .002"
from the sum of the bullet diameter and 2x wall thickness (that is for me .306+.0145x2-.001=.334 bushing size). OK, here is my
particular problem/issue: my new, unfired brass already has a neck O.D. of .333", meaning my I.D. is .304". So, does this mean
I should not neck size at all with the virgin brass? I know that once fired, the numbers will change dramatically, but what about that first loading
of unfired brass? No instructions or reloading manuals that I have read (and that's quite a few) mention this as a possibility- it seems that
they all assume that new brass will always need to be sized down for proper neck tension, and that makes me nervous.

 

[nhm16]

ALLRIGHTYTHEN! Didn't intend to open a can of worms with this thread- but hey, the more I read the more I learn!
So, a "standard" method for determining proper bushing size, and thereby neck tension, seems to be subtracting .001" to .002"
from the sum of the bullet diameter and 2x wall thickness (that is for me .306+.0145x2-.001=.334 bushing size). OK, here is my
particular problem/issue: my new, unfired brass already has a neck O.D. of .333", meaning my I.D. is .304". So, does this mean
I should not neck size at all with the virgin brass? I know that once fired, the numbers will change dramatically, but what about that first loading
of unfired brass? No instructions or reloading manuals that I have read (and that's quite a few) mention this as a possibility- it seems that
they all assume that new brass will always need to be sized down for proper neck tension, and that makes me nervous.

I would at least run an expander mandrell through it (after a good lubing to the inside of the case necks, of course) to make sure that all the necks are round and that the ID is more or less all the same.

That's all I do for most Lapua brass.

 

[DanConzo]

grig-You should always expand your neck on new brass to take the dents and wrinkles out, then you can do what you need to do with it. Put the proper id in it to fit your mandrel to neck turn or size down to the proper id and od (if you don't ont), trim or whatever you want to do.

 

[jlow]

LOL! Neck tension is always a can of worms if only because we are not sure what we are looking at!

OK, I thought you said your bullet diameter is .307 and not .306? If assuming it is .306, if you are dealing with unfired Lapua, it’s best to not assume all the new brass has the same neck tension, and you need to expand it first with a neck expander mandrel or it will have too much tension. Once you have expanded it, you can size it down to where you think it has to go – this is normal.

http://www.sinclairintl.com/reloading-equipment/case-preparation/neck-turning/turning-expander-mandrels/sinclair-expander-mandrel-oversized-prod33134.aspx

The way I look at it, “neck tension” is really a catch all term that does not mean one thing except the complex interplay between the bullet and the case. Certainly the difference in diameter of the bullet and ID of the case is one but simple things like lubrication (carbon in the neck or lack of), how smooth the surface of the neck/bullet is, degree of annealing/metal softness, bearing surface of the bullet in contact with the case, not to mention the dreaded “cold fusion”, and I seriously doubt that is even close to a comprehensive list…

 

[M14AMU]

And THEN....you've got this guy:

http://www.accurateshooter.com/guns-of-week/gunweek063/

Pay attention to his EXTENSIVE (Not) reloading procedures!

It ALL goes back to HOW MUCH trigger time, rounds fired, and barrels burned out to be able to sit in the winner's chair!!