Neck tension and max bullet grip force

[LaHunter]

I can say for 100 percent fact that reguardless of spring back, there is a huge difference when shooting groups with .002 NT or .005. This is being measured from outside neck before seating bullets. Certain guns like more. Certain like less.

Can someone explain the 'why' behind why this occurs? Given a certain brass, bullet, powder, primer combo, by changing only the neck bushing sized you can impact your groups. How do we know for 100% certain that the gripping force of the brass neck on the bullet doesn't increase by reducing the bushing size to say .005 or .006 vs .002 less than the loaded round neck o.d. Just wanting to get my brain wrapped around this.
Thanks for replies

 

[whirlwindjml]

I shoot for .001 tension now. My extreme spreads have lowered. Ymmv

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.

I use graphite to lube the necks to prevent the welding. I use old h1000 powder kernels in a musket cap can mixed with the graphite and dip before I charge the case with powder.

 

[ntsqd]

The question is really, where does the transition from elastic deformation to plastic deformation occur? Let's say that it happens between -.003" and -.004" That is to say that -.003" is on the left side of Point B and -004" is on the right side of Point B in the generic graph below.

If so, then let's say that you get 25 lbf of"grip" on the bullet at -.003" Since the metal in the neck transitioned into the plastic portion when a bullet was seated in a -.004" neck, it went past Point B. If it is at C then the grip on the bullet will be less than if it was at B because the metal in the case neck is now permanently stretched. Note that the slope of the line is shallower past C clear to D than it is between B and C or even between A and B. That shallower slope is saying that we're getting less grip for each diameter increment that we stretch the neck in seating a bullet.

 

[Mikecr]

Your spring back well displays the OA proportional limit.
Point B (Yielding) occurs under 1thou. This is the point and any beyond where brass does not fully recover on strain release (it's sizing).
When you seat bullet bearing into interference greater than OA proportional limit, you're upsizing for that bearing length, and getting little to nothing for tension force gain. IMO, with respect to benefit-vs-detriment, it's just excess sizing.
After all, you can tune with a reasonable/minimal neck sizing.

If your neck sizing length is greater than seated bearing(like FL sizing of necks), then you increase bullet gripping force(tension) a huge amount.
But nothing about this is free. The area brought into tension includes forces from donut and shoulder angle. The highest stress point (grip) is likely the base-bearing binding that you're causing with the non-expanded area below seated bearing.
This higher tension will come with a lot of variance.

Where extreme tension works, is competitive BR underbores. 6PPC, 30BR.
The attributes of these are nothing like that of hunting capacity cartridges, and we cannot viably gain from the extreme pressures these tiny cartridges can run, which they need high starting forces to obtain.
Maybe you have found benefit in FL sizing of necks. It's possible. But it's not something I would ever do.

 

[243winxb]

The question is really, where does the transition from elastic deformation to plastic deformation occur?

76,000 PSI + https://en.m.wikipedia.org/wiki/Plasticity_(physics)

modulus of elasticity- Cartridge Brass-
Material is 70 copper/30 zinc with trace amounts of lead & iron , called C26000. Material starts to yield at 15,000 PSI when soft (annealed), and 63,000 PSI when hard.
Material yields, but continues to get stronger up to 47,000 PSI when soft, and 76,000 PSI
when work hardened. Modulus of Elasticity is 16,000,000 PSI. This means to pull a 1.000 inch long strip to 1.001 inch long induces a 16,000 PSI stress.
So if you pull a 1.000 inch strip to 1.005 inch long, you get about 76,000 PSI, which is the max obtainable.

 

[Mikecr]

We can't currently measure the forces, and our application is not really a tensile test.
It would be calculated as hoop stress

 

[ntsqd]

When I said "where does the transition occur" I was talking about where in the distance of the neck expansion from sized to seated bullet, not where in the stress-strain curve. The condition of the neck metal (fully annealed? PArtly annealed? fully work-hardened?) as well as it's actual composition (is it 'pure' C2600 or does that particular case mfg spec their base metal doctored with some other element(s)?) obviously play a huge role in how the neck behaves.
With the shoulder buttressing one end of the neck it wouldn't be a simple Hoop Stress calc either. It probably could be accurately predicted in FEA, but a tiny dimensional error in the model would mean GI-GO. I suspect that you would have to model each and every case and test that metal for it's yield point to have valid numbers.

 

[Mikecr]

Well we need to actually measure expansion forces. Unfortunately, there is no way to do it right now.
Anything inside elastic ->is shown by full recovery springback (~1/2thou with 24cal necks).
So the transition from elastic deformation to plastic deformation' occurs at >1/2thou sizing..

 

[ntsqd]

I wonder if one could get a decent idea from using a modified version of the Porter Mandrel Expander (uses common gage pins instead of purpose-made mandrels) in an arbor press like the K&M press that is fitted with the (seating) force gauge? Can buy those pins by the .0005" diametrical increment. Could then plot the insert force vs. the pin diameter. I suspect that those cases would be sacrificial as getting them off of the gage pins may be difficult w/o damage.

 

[bedrok]

More from my experience. I believe weatherby calibers benefit from tighter neck tension. I've also heard it can prevent secondary pressure spikes

I've also found weatherby calibers to like .003 to .004 tension better than .001, sometimes drastically. But I've always wondered why

 

[Old Rooster]

bedrok I have found the same thing!
My 300 weatherby mag will not shoot as tight groups as a lightly crimped round will so I bought a Lee Factory Crimp Die and put light crimps on necks.
I bet I can reduce my groups with a bushing die a little tighter or maybe that rifle just likes the Lee F C D.
Since you posted that point I have decided to increase the neck tension to about .004 and check groups.
All my other rifles shoot with .001-.002 neck tension but that 300 weatherby mag likes them a bit tighter it seems.
Thanks for confirming what I thought.
Old Rooster

 

[ntsqd]

Instead of calling it "neck tension" I propose that it be called what it really is, Interference Fit (IF).
As has been discussed the grip on the bullet does not linearly increase with the change in neck internal dimension relative to bullet OD. A .001" change on a .224 bullet OD is not not the same change as a .001" change on a .375" OD bullet. Nor is a .002" change on any bullet OD twice the grip of a .001" interference fit. Using the term "neck tension" implies that neither of things occur or matter when they both occur and they both matter.

 

[ButterBean]

Instead of calling it "neck tension" I propose that it be called what it really is, Interference Fit (IF).
As has been discussed the grip on the bullet does not linearly increase with the change in neck internal dimension relative to bullet OD. A .001" change on a .224 bullet OD is not not the same change as a .001" change on a .375" OD bullet. Nor is a .002" change on any bullet OD twice the grip of a .001" interference fit. Using the term "neck tension" implies that neither of things occur or matter when they both occur and they both matter.

It was called IF to start with and the slang term Neck Tesion caught on and took over

 

[Old Rooster]

So if Interference fit is not increased with a smaller neck size one of problems will continue.
With a magazine full (3 rounds) the last one will have some bullet setback even with new brass with neck bushing and mandrel.
I used a Lee Factory Crimp Die and that stopped.I just assumed the interference fit was increased and thought I might increase the interference fit to stop a potential explosion.
I sure enjoy your posts as you a couple of others must be an engineer or retired.
Thanks in advance for your reply
300 weatherby mag
Old Rooster