Neck tension and max bullet grip force

Tiny Tim

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Good thread that addressed some of the same issues.

 

longestrange

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With this, ultimately, we need a tool to measure neck tension. Not hardness,, not seating forces,, but actual hoop stress per use dimension.
If it was simple though, somebody would have done it.
I don't think we'll see it..
I'm thinking you could get a pretty good idea of neck tension in the following manner, though my brain is a bit rusty:
Get a tube of red brass of appropriate diameter. Adjust thickness by sizing/machining on at least a part of the length.
Anneal it.
Attach one end with swage fittings to a hydraulic hand pump or similar with a quality pressure gauge in the line. Plug the other end with swage fittings.
Bleed the air out of the tube.
Put a dial diameter indicator or similar in the area of interest, or use a blade micrometer.
Increase the pressure and plot against diameter.
You can also turn the neck off an actual case and push/press it over the tube before doing this to get an idea of shoulder effects.
So:
hoop stress s (in a tube) = pr/t
Since p = st/r you will need about 1700 psi to yield brass with 20 ksi yield in a .308" plain tube.
 

243winxb

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https://discover.dtic.mil/

The military uses "Bullet Pull" The 5.56 minimum is 35 pounds.

E.1-4.3 Bullet Pull.
The purpose of this test is to determine, and/or confirm, the force required to extract (pull) a
projectile from its case. It is a measure of security of the projectile within the case so that it will
not separate during handling or feeding. Correct (and consistent) bullet pull force affects
consistency of propellant burning and resulting velocity consistency. Ammunition
specifications usually state a minimum extraction force that the projectile must resist.

Hold the cartridge case to be tested in a test fixture and apply a tensile load to the bullet at a rate
of travel of the loading head given in the ammunition specifications. The procedure and
equipment used should be reported along with the results and observations. Record the force
(average, range & standard deviation) required to extract the projectile from the case, the rate of
travel of the loading head, and the test apparatus used.

(2) Bullet pull is measured by inserting the complete round
of ammunition in a univeraal testing machine, a lover clamping device
holds the bottom of the cartridge case and an adjustable sensitive cross
head grips the projectile body between the rotating band and bourrelet«
Motion of the sensitive cross head at specified strain rates produces
hydraulic pressure in the loading cylinder of the machine, load indication
read directly on the dial of the test machine is a function of this by*
draulio pressure.
(3) In actual practice, hydraulic and pnetnatio pipe operated
by foot pedals are used to simplify and speed up insertion and removal
of the the teat rounds. The operator for safety reasons conducts the de«
bulleting teat by remote control from a barricaded room«

https://www.mecmesin.com/publications/bullet-extraction-and-seating-force-testing
 
Last edited:

Mikecr

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E.1-4.3 Bullet Pull.
The purpose of this test is to determine, and/or confirm, the force required to extract (pull) a
projectile from its case. It is a measure of security of the projectile within the case so that it will
not separate during handling or feeding.
We do need enough friction to keep bullet seating from changing.
Their other notion of function is hogwash. If necks did not expand and bullets had to be pushed through their friction, everything about our loads would be different today, as otherwise our guns would explode.
Necks are the first case expansion, into clearance provided, to release bullets.

longestrange, a testing rig would follow your line of thinking.
But, it would be of little use to merely test a representative sample. We need to test each actual case at a point where we can adjust and retest until reaching desired.
Similar to my approach with an instrumented mandrel die. I test 'pre-seating' forces, so that I can adjust each to desired -before seating actual bullets. This way all necks match before seating.
It works because I do not leave necks in so much interference that later bullet seating would change my numbers (no seating upsizing).
 

243winxb

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https://www.handloadersbench.com/fo...h-com-load-data/27215-neck-tension-experiment

Used a valve spring pressure tester.

"
OVERNIGHT test. or 24 hour test really

The bullet that took 1 extra firing and 70psi to seat
took 95 psi to pull it.
Another that took 95 psi to seat that i found laying there from the first rounds of testing.

That one had a few days to sit alone..It took 130 psi to pull it..
That one had .003" neck tension.

Now i measured all the necks today after pulling the rounds.
All have .001" or measure .307" id give or take a little on either side of the line on my calipers...so within .0002" "
BulletPullNeckTension.jpg SWAMPRATT.jpg
 
Last edited:

Hoppsing55

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I Think the answer you seek is with the materials, "modulus of elasticity".
Room temp values:
Copper Alloy C26000 (cartridge brass)GPa =11010-6psi /16
 

Baboltin

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Dissimilar metals can react and switch electrons and eventually "weld" to one another. Galvanic corrosion, electrolyic corrosion, electrolysis are other terms for it. There are many factors that come in to play with neck tension. If you have ever used an ultrasonic cleaner for you brass the bullets almost have to have lube on them, they actually squeaked going in and used way too much seating force, so I don't do that anymore. There is a point of no more tension but it depends on so many other things, mainly the brass.
So with this being said, I shouldn’t clean my brass in an ultrasonic cleaner? I have a bunch of one shot nosler brass for my 7 mag that I am doing brass prep on and when I originally reloaded them I never used neck lube or anything just seated the bullet into the brass. Should I use lube? And if I shouldn’t use a ultrasonic cleaner then what does everyone recommend to clean brass with? I also have a nut shell media tumbler.
 

Baboltin

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Also I apologize I am not trying to steal the post, I am hopeful to get a quick answer that was a thought while reading this post about neck tension.
 

MagnumManiac

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This is my understanding of what occurs upon firing a cartridge through testing with firing a cartridge with just a primer in an oversized barrel.
Firstly, the primer pressure is enough to expand a neck and the bullet is fired at quite some velocity out of the oversized barrel.
Upon measuring the neck before firing and after, it is obvious that the bullet is not held for any length of time other than the time it takes for the neck to expand. There are no drag marks on the bullets surface, which indicated very little, if any, movement occurs prior to the neck expanding.
The neck would move on average .001”-.0015” larger, which is why I believe the spring back amount is all the bullet is held with.
I also tried crimping the bullet, 9 out of 10 bullets failed to straighten the case mouth completely and all were faster than non-crimped bullets.
This is why factory bullets are crimped, it increases start pressure and reaches a gentler curve under MAP, hence why factory fodder works in so many different firearms.
So, the conclusion is this:
Bullets do not get held upon firing as many think. As soon as the bullet is released of the neck, there is no tension, the neck simply expands and the bullet is free to travel. A crimp delays this by milliseconds as the brass takes longer to expand and release the bullet. There is virtually zero bullet movement before the neck expands enough, maybe a couple of 10 thousandths, and the bullet is released.
When you pull bullets, you can feel that first resistance, you can also feel that the tension has changed as soon as the bullet moves. If you re-seat a bullet in a case that has had a bullet pulled from it, you also see that the neck tension is not the same.
The only difference to this in a guns chamber is the gas pressure forcing the neck into the space provided.

Anyway, that’s my own conclusion from testing my theories. As I have had bullets do strange things and wanted to know why.

Cheers.
 

ntsqd

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I'm barely wrapping my head around this in a way that I can put into words. It seems to me that the fundamental problem is that the Modulus of Elasticity is a moving target. Degree of work-hardening, number of times fired, temperature and saturation time of the most recent annealing, chemical exposure, specific alloy, and probably one or more things that I missed all have an influence on it.

Next, that is just the Modulus of Elasticity and not the force that the case grips the bullet with. The transition point from elastic (will recover to original dimension(s)) to plastic (will not recover to original dims) does not appear to be fixed. Seems to me that a desired destination would be to be able to say that, for example, "Norma brass case necks will transition to plastic deformation at 100.25% of the relaxed diameter, while Win brass case necks transition at 101.00%" although I suspect that this will be a lot by lot result rather than being merely brand specific.

Been a really long time since I last calculated hoop stress, but assuming that we stay within the elastic region I think that it can be calculated for each specific instance and frm there we could know the pressure on the bullet. Equal and opposite forces and all of that. The problem is that if the brass in the case neck transitions into plastic deformation during the seating operation then the calc is junk and meaningless. To avoid that we need to know a lot more about the brass than we do now.

Based on Magnum's post above and what MikeCR has said about this in the past I suspect that every seating operation exceeds the elastic limit making a hoop stress calc a waste of time. If consistency is the goal then I suspect that it is more important to know how much the brass will spring back after pulling a seated bullet. The problem is measuring this. The Uncertainty Principle rears it's ugly head here. Can't measure the system without disturbing it beyond use, and if we can't use the exact system that we measured then how exact are the results?
 

Mikecr

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When you pull bullets, you can feel that first resistance, you can also feel that the tension has changed as soon as the bullet moves. If you re-seat a bullet in a case that has had a bullet pulled from it, you also see that the neck tension is not the same.
There are always direct reasons for pushing or pulling frictions to change. Luckily, it has no affect on actual bullet tension.
When we upsize with bullet seating the forces are higher than could be calculated as the bullet-base junction rolls brass outwards.
Our sizing is angular, and is most commonly seen when bushing neck sizing greater than 5thou in one step. This rolls brass inwards, beyond stamped on the bushing.
Anyway, while seating, that brass rolls outward and curves back onto the bullet bearing. It is a very slightly bowed grip. The greater the sizing, the greater the bowing energy added. If friction were normalized between, a 2nd seating(without that upsizing) would be way easier, but trust me on this: tension is still the same.
With either the 1st or 2nd seated bullets, having a slightly bowed grip, if you run a pull test the force can be higher than would be expected as the bow is taken out. It's a Chinese finger trap effect. It's not always what you have, but it does explain differences seen in past pull tests.

On friction alone, the only reason we should strive for reasonable seating force is for consistent CBTO (without a battle for it).
Greater/varied seating forces cause wedging changes with the seater plug on bullet noses.
It's ok to use a dry film lubricant in new necks. Other than moly, it won't change anything over a fired carbon layer, and won't affect MV.
Do not use a wet lube..
Moly does affect MV (lowers it), but this is NOT due to reduced friction. Moly is special in that it cools the charge with it's phase change.
A latent heat of vaporization, that other dry films do not introduce.
 

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