When does neck tension cease to matter ?

[JimSC]

I was doing some thinking on neck tension and came across Damon Cali's article

Case Neck Tension - A Stress Analysis

which got me to wondering about at what point does the neck tension cease to matter in the firing sequence. The firing pin ignites the primer, the primer ignites the powder, pressure builds and at some point the bullet moves forward and the case expands. As the neck brass expands at some point the neck is no longer gripping the neck. Gas continues to flow out of the case forcing the bullet into the throat and down the barrel.

Bryan Litz in his book Modern Advancements in Long Range Shooting Vol II, chapter 6 Neck Tension experimented with varying neck tension from .001 to .003 and found that while increasing neck tension had only single digit reduction on muzzle velocity in the .223, and .243 tests the SD was slightly improved with increased tension using .003 neck tension over .001. The SD reduction only came into play for the .223 and .243, the .308 was unaffected except for a 2FPS increase with the additional neck tension

Damon in his article gets into the yield point of annealed vs unannealed cases and makes a valid point that with work hardening the elastic range of the metal decreases and annealing extends it. What I am wondering is does it matter. The yield point only comes into play as the neck expands and by the time it is reached the case neck has expanded .002 or .003 and is no longer in contact with the bullet. Does the yield point really matter at that point since it is no longer in contact with the bullet?

Anyway this all got me to thinking about why the .223 and .243 behaved one way and the .308 another and at what point in the firing sequence the neck ceases to matter. Just something to ponder on a cold February day

 

[rsmithsr50]

IMHO when jamming it has less affect.
before jam it increases start pressure by holding the bullet.
so when jumping neck tension is a factor

 

[dellet]

When the bullet is fully supported by the bore, and the neck seals the chamber from allowing gasses to blow back.

The first basic function of the neck is to center the bullet down the bore, the second is to seal the chamber and keep all the pressure moving forward behind the bullet.

Neck tension, case elasticity will effect start pressure, which in turn effects the timing of when those two things will happen.

Then you get into the whole jump or jam variable.

 

[lpreddick]

I've spent a lot of time and shooting to realize some things about " neck tension". First, it's a misnomer. We use a measure of difference between loaded and unloaded neck diameter as " neck tension". .002 difference is common. .002 in an annealed case is not the same "grip" on a bullet as it is in a neck not annealed. .002 in a very thin neck does not have the same "grip" as in a thick neck. This " grip" has to be overcome by case pressure before the bullet will move. When i jumped bullets, neck tension/grip had a greater influence on velocity, therefore accuracy. The " grip" of a newly loaded case might not be the same 6+ months later...cold bonding of bullet to neck can occur and result in a greater " grip" and therefore require greater case pressure to move that bullet...This results in a greater muzzle velocity, reduced barrel time and change in point of impact on target. If one jumps bullets, neck tension needs to be as consistent as possible.
Now I seat bullets well into the lands and believe muzzle velocity and accuracy is more consistent. With the bullet .020+ into the lands, I believe the case pressure required to move that bullet further down the bore far exceeds that needed to break the neck " grip". Neck tension( grip) must be sufficient to prevent the bullet being seated deeper as it engages the lands. .002 is sufficient for my barrels if case necks are thick, while .004-.006 is needed in really thin necks.

 

[Howland]

My theory - which I don't have the resources to prove or disprove - is based on smokeless propellant burning progressively, i.e., the higher the pressure, the faster it burns.

I was working up a .338LM load and my most accurate loads were with a low powder charge, not too far over start charge, but ES was horrendous. I bought a set of 21st Century mandrels in 0.005" increments and repeated OCW which showed the 0.003" interference fit consistently better groups than others across the charge weight spectrum but still the most accurate charge had very bad ES, over 200!

I'm convinced that it was the low powder charge that caused the poor ES and not insufficient neck tension as I had originally suspected and went with a higher charge that was almost as accurate but ES was excellent and impact points little changed from adjacent charge weights.

Still, the 0.003" interference fit outperformed other neck size dimensions. When the primer first ignites the powder, that initial resistance from the neck tension influences the rest of the ignition sequence and contributes to or detracts from consistency but likely it is way down on the list of influences.

But we're all OCD here.

 

[JimSC]

I've spent a lot of time and shooting to realize some things about " neck tension". First, it's a misnomer. We use a measure of difference between loaded and unloaded neck diameter as " neck tension".

I agree, I prefer the term neck friction. In particular when people use "seating force" as a guide. I have never seen these tester clean the necks and bullets before the test. A minute amount of skin oil would throw the measurement off. So would a film from burned off contaminants from an annealing process. As with annealing I would love to see some real scientific testing done using a double blind method and performed by a neutral third party testing agency

 

[Richard Coody]

When does neck tension cease to matter ?

Maybe this answers.your question.
theoretically neck tension matters a lot. But not as much when the firearm being used lacks the ability to show that difference.

 

[Richard Coody]

I agree, I prefer the term neck friction. In particular when people use "seating force" as a guide. I have never seen these tester clean the necks and bullets before the test. A minute amount of skin oil would throw the measurement off. So would a film from burned off contaminants from an annealing process. As with annealing I would love to see some real scientific testing done using a double blind method and performed by a neutral third party testing agency

Well my experience was not scientific at all but i learned a lot about neck tension with a 21st century hydroseater. My little savage 6br shoots pretty accurately using loads with a very consistent seating force. Too low or too high and the group size can open up close to a tenth. Now i leave carbon in the necks. Just lightly brush them with a nylon brush. Don't do anything special to bullets.

 

[SPQR70AD]

I never understood how neck tension can be the same for a bunch of rounds when I seat bullets none feel the same as the neck grabs them I think rifles seem to shoot better when touching the lands is because they release at the same time. I never liked that being there is to much pressure and wears the barrel out sooner so I use the Lee factory crimp die set the length to factory size. I do not bench shoot but from 200 -500 yds I have shot tiny groups that way with different calibers and rifles

 

[DaveTooley]

Sometime when your at the range recover some bullets from the berm. Look at the land engraving pattern. You'll find there is contact on top of the lands and about 50% of the groove and then the base obturates to form a seal. Wear patterns in the barrel can be easily seen confirming this. So my assumption is the neck maintains contact until the bullet leaves the case or very nearly so.

 

[AlNyhus]

Sometime when your at the range recover some bullets from the berm. Look at the land engraving pattern. You'll find there is contact on top of the lands and about 50% of the groove and then the base obturates to form a seal. Wear patterns in the barrel can be easily seen confirming this. So my assumption is the neck maintains contact until the bullet leaves the case or very nearly so.

Agreed, Dave. Peak pressure drops off rapidly after ignition and the necks want to return to their original diameter. That also explains why in certain scenarios, a bullet won't fit back into the neck of a fired case, even though there is adequate neck clearance.

Good shootin'. -Al

 

[ebb]

I was told by a national champion short range benchrest shooter that if the bullets were seated to the lands neck tension meant little to nothing. Is this true? From your statement above maybe not?? I always thought the case opened up before the bullet moved if loaded in the lands ?

 

[JimSC]

Well my experience was not scientific at all but i learned a lot about neck tension with a 21st century hydroseater. My little savage 6br shoots pretty accurately using loads with a very consistent seating force. Too low or too high and the group size can open up close to a tenth. Now i leave carbon in the necks. Just lightly brush them with a nylon brush. Don't do anything special to bullets.

Honestly Richard I think what your rifle and load does at the range is all that matters. Savage 6BR , Criterion barrel ?
Here is a 11 shot group from my Savage 6BR at 300, cases on 5th firing, no annealing, no brushing, no neck lube just dry tumbled, sized, then necks trimmed and seated on my humble Lee turret

 

[bozo699]

[QUOTE="Richard Coody, post: 37719906, member: 1302334"Now i leave carbon in the necks. Just lightly brush them with a nylon brush. Don't do anything special to bullets.

[/QUOTE]
I totally agree with this!
Wayne

 

[heydogman]

I recently saw the effects of over annealed brass. Neck seating force went from 60 to 3. 0025 "neck tension".Going down a bushing size did not help. Thinking, since the 338 LM has much more surface area for heat transfer I added one second more time than the 6BR. The heat really didn't transfer.

 

[Laurie]

As with annealing I would love to see some real scientific testing done using a double blind method and performed by a neutral third party testing agency

Bryan Litz's book Modern Advancements in Long Range Shooting Vol II starts to get into these subjects as mentioned above by @JimSC. Sadly, AB had only just acquired an AMP annealer shortly before experimentation had to be closed down to meet the book's publication deadline, so annealing effects were only given modest attention. (However, the results Bryan did achieve in the limited test programme won't delight those who've spent large sums on expensive induction annealers.)

On the question as to why some cartridges appeared to see differences and others didn't from differing degrees of tension, I suspect that as the changes were small, they may simply not have been significant enough to produce across the board trends in every cartridge's particular combination of circumstances - pressure and pressure build-ups, charge sizes, bullet accelerations etc. Note too that these tests only look at chronograph readings and the stats they generate. Whilst reducing ES and SD values may be desirable we all know that actual results on the paper aren't necessarily related to how good these metrics look. There appear to me to be many possible routes to good results on paper and I suspect that having found one that suits you, your cartridge's characteristics and your rifle, then continuing to maintain it consistently is the most important thing rather than whether x amount of tension works better or worse than y across the board.

The other point of interest from this section of the book was that IIRC Bryan started out with a fancy bullet seater set up with a pressure gauge to ascertain neck tensions through that route, but gave up on it as he found that readings were overly affected by how one operated the press. His view is that no matter how consistent the operator tries to be, the human factor makes it an unreliable indicator of the degree of neck tension present and its variability. I'm sure that this will produce howls of outrage from satisfied users of such presses, but I suspect that he is correct in that it would need a consistent force powered operation to be really useful. (And even then, what about variabilities in neck and bullet surface coatings and the need to get into cleaning / decontamination efforts to eliminate them as mentioned in an earlier post - some 10 steps too far in my reloading regime's acceptable level of detail, workload and complexity!)

 

[JFrank]

Good thread’ @JimSC

I’m much the same as Richard, leave the carbon, brush lightly, bushing is .003 under a loaded round seems to shoot the smallest and most repeatable.
I don’t anneal brass at least for now, I do feel that the less I mess the better it shoots.
Go figure
J

 

[heydogman]

Ten at 300, prone using mirage during the typical 11 AM Orangeburg switches. Cases wiped clean, neck lightly brushed . annealed with a oil lamp. Seated force was within 3 lbs for all 10. Shot #8 the low shot I'm blaming on Fire Ants that traveled all the way up my pant leg. I went up one click and shot 9 and 10,the high ones then headed to the latrine fighting ants. Happy with the vertical and will continue to get the seating force the same.

 

[dellet]

This is a really complex/confusing/confounded subject because the number of variables and type of shooting can change the questions as well as the answers. Some of the problem is people don't really step out of there favorite discipline and see how others deal with the same problems.

My earlier post was my opinion of the very basics of the purpose of the neck in the first place, if you don't define the purpose, it's impossible to know when it stops being relevant. The basic purpose of the neck is to hold the bullet in such a way that it starts down the bore. That's why so much detail is given to keeping the neck parallel to the body, perpendicular case head as well as centered and concentric.

The the next problem is a clean, crisp, even push down the bore. It does not matter how centered the cartridge in the chamber is, if ultimately you try to push it in sideways. An easier example to see this play out is in archery. It just does not matter how straight and perfect everything is lined up, a poor release pushes the the arrow out of line because the string moves off center. Using a mechanical release instead of fingers, has about the same effect on group size as truing primer pockets and flash holes.

A clean, crisp, consistent release of the bow string not only increases velocity, but also lowers the ES/SD numbers through a chronograph. I would suggest neck tension, bullet hold, is all about finding a better release.

What happens if you eliminate neck tension altogether? or for that matter how about we eliminate the entire neck?

Jacketed bullets have only been around about 100 years. Target shooting has been around closer to 500. The fundamental problems of accurate shooting have not changed, only the tolerances and methods of getting the bullet into and out of the bore in a straight line and to continue a straight path to where you pointed it.

100 years ago the thought was basically if you align all of the components up the same way every time, putting the manufacturers defects in the same place every time, the bullet will go the same place every time.

In many matches the same case was used for every shot and indexed to the chamber. Any neck sizing that was done was to flair the neck so it would seal the chamber behind the bullet. Without this flair, the case would often collapse due to pressure between it and the chamber.

Neck tension was not an issue because because the bullet was already seated in the bore. The debate was if there was better accuracy when it was started from the muzzle or the breech. Seating from the muzzle keeps the little fins from forming on the base when the lead was smeared back from the lands if seated from the breech.

Seating depth was adjusted for accuracy. Just enough of the base was left out of the bore to control obturation. Enough bullet left for expansion to seal the gasses, not so much that the base would deform and no longer be square to the bullet. Bullet hardness, powder charge, chamber pressure factored in how far ahead of the case the bullet was seated.

A single case fire formed to the chamber and indexed has been replaced with chamber and case tolerances measured out 4-5 decimal points. Cast bullets whose weight and shape changed with mold temp and alloy, that were also marked in the mould so they could be indexed into the chamber, have been replaced with bullets held to tighter tolerances than some chambers.

If you have a perfect bullet, loaded into a perfect case, in a perfect chamber, leading into a perfect bore powered by a perfect charge that was ignited perfectly, you have one variable left. A perfect release of the bullet.

The old guys gave up and eliminated that variable by taking the bullet out of the case, that doesn't mean they did not recognize the problem, more likely it means they did.

The new guys are giving it another try. How much interference fit you need will likely vary cartridge to cartridge much the same way and for the same reason as the seating depth into the bore varied 100 years ago. Starting pressure, elasticity of the brass, chamber clearance, bullet design will all probably require different "tension".

One last thought.

Fouling shots are to season a clean bore to increase shot to shot consistency.

Why does most everyone agree that a bullet needs a seasoned bore to be consistent, but the idea of a seasoned neck drives most reloaders to the curb?

 

[tom]

My experiences, with rifles that are capable, say it matters jumping or jamming. I've seen it be the difference between a gun that shoots small most days, and one that does every day. I've seen it add to the powder window width, both on paper and velocities. It's like everything else, it needs to be part of your testing/tuning on every barrel, unless you can afford to keep buying barrels until one chooses your pet load.


Tom