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Measuring neck tension

Campbell

A good few sleeps ago I read somewhere about measuring neck tension where the thickness of the neck wall thickness was brought into the equation.

I can't now find any references to that particular method. Have any of got links any link(s)?

I have read the method whereby one measures the diameter of a resized case, then measures again when the bullet is seated.

It's driving me crackers trying to find the method whereby the neck wall thickness is part of the calculation!!

Many thanks in advance
 
Hanbleceya said:
Yes. And it only took one post to get there. Good job.

Campbell, you'll get there. Try just getting away from it for a few minutes and thinking it through without cases and gauges and books, etc.
Click to expand...

Hi Bud

Many thanks , but the link does not formulate the calculation of actual neck tension. (unless I am too tired to see it)

My understanding is that to choose a bushing one needs to: -

Measure neck wall thickness x 2
Add diameter of bullet
Subtract desired neck tension.

This only gives a theoretical neck tension using the wall thickness, I have read another method to ascertain actual neck tension (in term of diameter) using the wall thickness in the equation.

Many thanks again.
 
Campbell said:
Hi Bud

Many thanks , but the link does not formulate the calculation of actual neck tension. (unless I am too tired to see it)

My understanding is that to choose a bushing one needs to: -

Measure neck wall thickness x 2
Add diameter of bullet
Subtract desired neck tension.

This only gives a theoretical neck tension using the wall thickness, I have read another method to ascertain actual neck tension (in term of diameter) using the wall thickness in the equation.

Many thanks again.
Click to expand...
That's the way I do it, then ladder load for bushing size.
 
yes this is setting tension based on case neck DIAMETER not on mechanical TENSION.
I never seen a tension measurement based on DIAMETER. it would be valid in a single case as material. thickness, dia would all be variables.
 
Campbell said:
Hi Bud

Many thanks , but the link does not formulate the calculation of actual neck tension. (unless I am too tired to see it)

My understanding is that to choose a bushing one needs to: -

Measure neck wall thickness x 2
Add diameter of bullet
Subtract desired neck tension.

This only gives a theoretical neck tension using the wall thickness, I have read another method to ascertain actual neck tension (in term of diameter) using the wall thickness in the equation.

Many thanks again.
Click to expand...
I get that. I am trying to keep from skimming the necks on one tight walled build but my Lapua has even been showing more thickness variance than I like. ...just thinking out loud. Anyway, if one is striving for best results, it would behoove to turn the necks anyway so when you take case neck thickness and double it you won't be averaging say .012 and .014.
 
It does, actually, include the neck thickness and i would look at this from another perspective. 1) yes, it is a theoretical and, 2) i would not measure the neck diameter until you have a round seated in it.
compare the neck diameter of the chamber with the loaded neck diameter of the rounds ( average) and then decide to turn or ream necks accordingly. Relax, i'm a mechanical engineer and had to learn to accept the nearly-perfect, but not-quite, dimensions of brass, bullets, etc. once i loosened up, it all came together. LOL
 
The industry standard (SAAMI) is the force needed to push or pull a bullet from the case neck. Military specs use extraction force. This is what the burning powder has to overcome. 7.62 and 5.56 service ammo has 45 to 60 pounds extraction forces. 7.62 match ammo, 20 pounds minimum. Ive used a couple to about 15 pounds pulling bullets from inverted cases weighing the collet bullet puller and bags of bullets attached. I think this gives a better measurement than diameter differences

A given interference fit (diameter difference) will have different forces needed due to metal hardness and friction.

That said, a .0005" interference fit may be good. I've used less. Die necks .001" to .002" smaller than loaded round necks usually works. There's a bit of case neck springback. The case neck springs back a few ten-thousandths from the sized diameter.
 
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Very interesting data, thanks for sharing.

How was measured psi done? Tools, gauges? It's psi numbers don't seem reasonable.

If it takes 20 pounds of force to push a 30 caliber bullet out of a loaded case, it takes about 270 psi in the case to push it out. 135 psi for a 10 pound force needed. Bullet has about .075 square inch cross section area. 7.5% of a square inch pressure is all that's put on it.

Bullet seating force doesn't matter. Measure bullet pull a day or two after seating bullets, unless you shoot them within a few minutes
 
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I think you're making a simple problem difficult. When you are trying to be accurate within .001", and you measure 3 things to .001" with an error of .0005", then you add them up and your error bars are .0015"... problem.

Load a dummy round, mic it, subtract the desired neck tension, order your bushing.

--Jerry
 
Hi Folks

Many thanks for your replies.

I have got confused! Using the wall thickness of the neck is only useful in the calculation in order to determine the choice of size of the bushing die required. i.e: -

Neck wall thickness's = .00145 x 2 = .029
Add Bullet diameter = .308
Less desired neck tension ( .001)
-------
Bushing die required .336
====

The actual neck tension is determined by measuring the o/s diameter of a resized case, followed by the measuring of the o/s diameter of the same case with a seated bullet and the difference is the actual neck tension.

I think that I have had a senior moment and mixed up two arguments?
 
Campbell said:
The actual neck tension is determined by measuring the o/s diameter of a resized case, followed by the measuring of the o/s diameter of the same case with a seated bullet and the difference is the actual neck tension.

I think that I have had a senior moment and mixed up two arguments?
Click to expand...
Tension implies force, doesn't it?

Different fit diameters is called an "interference fit" everywhere else.
 
Bart B. said:
Tension implies force, doesn't it?

Different fit diameters is called an "interference fit" everywhere else.
Click to expand...

Completely agreed!

The term neck tension in this context merely gives a guide of the force exerted on the bullet by reference to the internal diameter of the resized case. AND it assumes consistent wall thickness and consistent metallurgical quality of the cases.
 
Bart B. said:
Tension implies force, doesn't it?

Different fit diameters is called an "interference fit" everywhere else.
Click to expand...

Bart this is another clear example of why we desperately need a glossary to avoid the problems and misconceptions caused by using an perpetuating the current slanguage we otherwise educated folks use on this and other forums.

I'm too old to manage such a project but would consider helping (review proposed entries maybe).
 
Fred Bohl said:
Bart this is another clear example of why we desperately need a glossary to avoid the problems and misconceptions caused by using an perpetuating the current slanguage we otherwise educated folks use on this and other forums.

I'm too old to manage such a project but would consider helping (review proposed entries maybe).
Click to expand...

A superb term! I have not read this before.:):D
 
Campbell said:
The actual neck tension is determined by measuring the o/s diameter of a resized case, followed by the measuring of the o/s diameter of the same case with a seated bullet and the difference is the actual neck tension.
Click to expand...
Will the tension force gripping the bullet be the same for all neck wall thickness and hardness?

The linked article on neck tension in post #2 finishes with:
True bullet grip is a more complicated phenomenon, one that is affected by numerous factors, some of which are very hard to quantify.
Click to expand...

It is easy to measure the force needed to get the bullet out of the case neck. It is far from being a phenomenon and simple to quantify.
 
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